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Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
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http://www.archive.org/details/textbookofcomparOOmill 


A  TEXT- BOOK   OF 
COMPARATIVE   PHYSIOLOGY 


FOR  STUDENTS  AND   PRACTITIONERS  OF 
COMPARATIVE    (VETERINARY)    MEDICINE 


BY 

WESLEY    MILLS 

M.  A.,    M.  D.,   D.  V.  S. 

PROFESSOR  OF  PHYSIOLOGY  IN  THE  FACULTY  OF  HUMAN  MEDICINE  AND  THE  FACULTY 

OF    COMPARATIVE    MEDICINE    AND    VETERINARY    SCIENCE    OF    MC  GILL 

UNIVERSITY,    MONTREAL  ;    AUTHOR   OF  A  TEST-BOOK 

OF   ANIMAL   PHYSIOLOGY,    ETC. 


WITH  476  ILLUSTRATIONS 


NEW  YORK 
D.  APPLETON  AND  COMPANY 

LONDON:  CAXTON  HOUSE,  PATERNOSTER  SQUARE 

1890 


Copyright,  1890, 
By  D.  APPLETON  AND  COMPANY 

All  rights  reserved. 


PREFACE 


Some  years  of  contact  with  students  of  comparative  (com- 
monly called  veterinary)  medicine,  and  a  fair  knowledge  of 
the  actual  needs  of  the  practitioner  of  this  department  of  the 
medical  art,  have  convinced  me  that  the  time  has  fully  come 
when  the  text-books  of  physiology  provided  for  students  of 
human  medicine,  and  which  the  former  classes  have  hitherto 
been  compelled  to  use,  should  be  replaced  by  works  written 
to  meet  their  special  wants  and  possibilities.  In  fact,  so  dif- 
ferent from  man  are  most  of  the  animals  which  the  veterina- 
rian is  called  upon  to  treat,  and  therefore  to  understand,  in 
health  as  well  as  in  disease,  that  only  the  absence  of  suitable 
works  of  a  special  character  can  justify  the  use  of  those  that 
confessedly  treat  of  man  alone. 

Unfortunately,  till  within  the  past  year  the  English-speak- 
ing student  of  comparative  medicine  has  been  without  a 
single  work  in  his  own  language  of  the  special  character  re- 
quired. Within  that  period  two  have  appeared — the  excel- 
lent but  ponderous  Physiology  of  the  Domestic  Animals,  by 
Prof.  Smith,  and  my  own  Text-Book  of  Animal  Physiology. 
It  has,  therefore,  occurred  to  me  that  a  somewhat  smaller 
work  than  the  latter,  embodying  the  same  plan,  but  with 
greater  specialization  for  '  .ie  domestic  animals,  would  com- 
mend itself  to  both  t>  students  and  the  practitioners  of 
comparative  medicine.  In  my  other  work  I  have  endeavored 
to  set  before  the  student  a  short  account  of  what  has  been 
deemed  of  most  importance  in  general  biology ;  to  furnish  a 
full  account  of  reproduction ;  to  apply  these  two  depart- 
ments throughout  the  whole  of  the  rest  of  the  work ;  to  bring 

374^  •  . 


iv  COMPARATIVE  PHYSIOLOGY. 

before  the  student  enough  of  comparative  physiology  in  its 
widest  sense  to  impress  him  with  the  importance  of  recog- 
nizing that  all  medicine  like  all  science  is,  when  at  its  best, 
comparative ;  and  to  show  that  the  doctrines  of  evolution  must 
apply  to  physiology  and  medicine  as  well  as  to  morphology. 

Comparative  medicine  is  essentially  broad.  It  will  not  do 
to  measure  all  the  animals  the  veterinarian  is  called  upon  to 
treat  by  the  equine  standard.  This  has  been  too  much  the 
case  in  the  past  for  the  good  even  of  the  horse  himself ;  while 
others,  that  fall  to  the  practitioner's  care,  like  the  dog,  have 
been  much  neglected  and  misunderstood. 

There  is  no  more  reason,  theoretically,  why  the  veterina- 
rian should  overlook  man  than  that  the  practitioner  of  human 
medicine  should  disregard  the  lessons  to  be  learned  from  our 
domestic  animals ;  hence  the  attempt  has  been  made  to  exclude 
references  to  the  human  subject  from  the  volume.  The  stu- 
dent of  comparative  medicine  may  learn,  by  careful  observa- 
tion on  himself,  to  understand  much  that  would  otherwise 
never  become  realized  knowledge ;  and  this  conviction  has 
been  at  the  root  of  a  large  part  of  the  advice  given  the  stu- 
dent as  to  how  to  study  throughout  the  work. 

All  that  relates  to  reproduction  and  breeding  is,  in  these 
days  of  vast  stock  interests,  of  so  much  practical  importance, 
that  on  this  account  alone  the  fullest  treatment  of  the  subject 
seems  justifiable.  But,  apart  from  this,  it  has  become  clear  to 
me  that  function  as  well  as  form  can  be  much  better  and 
more  easily  grasped  when  embryology  is  early  considered. 
This  I  have  tested,  with  the  happiest  results,  with  my  own 
classes.  Usually  those  taking  up  physiology  for  the  first 
time  are,  of  course,  not  expected  to  master  all  the  details  of 
embryology,  but  the  main  outlines  prove  as  helpful  as  inter- 
esting ;  nevertheless,  it  is  my  experience  that  a  considerable 
number  of  first-year  men  are  not  content  to  be  confined  to 
the  merest  rudiments  of  this  or  any  other  department  of 
physiology. 

That  a  work  written  on  so  new  a  plan  as  my  Text-Book 
of  Animal  Physiology  should  have  met  with  a  reception  al- 
most universally  favorable,  both  in  Britain  and  America,  in 


PREFACE,  v 

so  short  a  space  of  time,  encourages  me  to  hope  for  one 
equally  favorable  for  this  book,  which  is  offered  to  a  pro- 
fession from  which  I  look  for  great  things  in  the  interests 
both  of  man  and  the  lower  animals  within  the  next  few 
years.  The  time  has  certainly  come  when  medicine  must 
leave  the  narrow  ruts  within  which  it  has  been  confined,  and 
become  essentially  comparative.  To  hasten  that  consumma- 
tion, so  devoutly  to  be  wished,  has  been  the  object  with  which 
both  my  earlier  and  the  present  work  have  been  written.  Un- 
less the  student  is  infused  with  the  broad  comparative  spirit 
in  the  earliest  years  of  his  studies,  and  guided  accordingly, 
there  is  no  sure  guarantee  of  final  success  in  the  widest  sense. 
My  publishers  again  deserve  my  thanks  for  the  efforts 
they  have  made  to  present  this  work  in  their  best  form. 

Wesley  Mills. 

Physiological  Laboratory,  McGill  University, 
Montreal,  Canada,  August,  1890. 


CONTENTS. 


PAGE 

General  Biology        1 

Introduction 1 

Tabular  statement  of  the  subdivisions  of  Biology  ...         4 

The  Cell 5 

Animal  and  vegetable  cells 5 

Structure  of  cells  ..........         5 

Cell-contents         .        .         .         . 7 

The  nucleus 8 

Tissues 8 

Summary 9 

Unicellular  Organisms  (Vegetable)    .......         9 

1.  Yeast .9 

Morphological         .........         9 

Chemical        .         .  .         .         .         ,         .         .         9 

Physiological  .         .         .         .         .         .         .         .         .10 

Conclusions 10 

2.  Protococcus .         .11 

Morphological        .         .         .         .         „         .         „         .         .       12 

Physiological „         .         .         .12 

Conclusions    .         ....  12 

Unicellular  Animals .         .         .         .         .         .         .         .         .         .13 

The  proteus  animalcule 13 

Morphological 13 

Physiological  .         .         .         .         .         .         .         „         .13 

Conclusions    .  •  .         .         .         .         .  ■  „         .15 

Parasitic  Organisms  .         .         .         .         .         .         .         .         .15 

Fungi 15 

Mucor  mucedo        .         .         .         .         .         .         .         .  17 

The  Bacteria 18 

Unicellular  Animals  with  Differentiation  of  Structure       .         ,         .       21 

The  bell-animalcule 21 

Structure  .  21 

Functions 23 


Vlll 


COMPARATIVE  PHYSIOLOGY. 


Multicellular  Organisms    .         . 

The  fresh-water  polyps  ....... 

The  Cell  reconsidered       ....... 

The  Animal  Body — an  epitomized  account  of  the  functions  of  a 

mal 

Living  and  Lifeless  Matter — General  explanation  and  compari 
their  properties  ....... 

Classification  of  the  Animal  Kingdom       .... 

Tabular  statement         ....... 

Man's  place  in  the  animal  kingdom        .... 

The  Law  of  Periodicity  or  Rhythm  in  Nature — Explanation 

illustrations 
The  Law  of  Habit    . 
Its  foundation 
Instincts       .... 

The  Origin  of  the  Forms  of  Life 
Arguments  from : 
Morphology 
Embryology 


3  and 


Mimicry     . 

Rudimentary  organs . 

Geographical  distribution 

Paleontology     . 

Fossil  and  existing  species 

Progression 

Domesticated  animals 
Summary 
Reproduction 
General 
The  ovum 
The  origin  and  development  of  the  ovum 

Changes  in  the  ovum  itself 
The  male  cell 

The  origin  of  the  spermatozoon 

Fertilization  of  the  ovum  . 
Segmentation  and  subsequent  changes 

The  gastrula     .        .        . 

The  hen's  egg   . 

The  origin  of  the  fowl's  egg 
Embryonic  membranes  of  birds 
The  fuital  (embryonic)  membrane 

The  allantoic  cavity  . 

The  placenta 

The  discoidal  placenta 


of  mammals 


PAGE 
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23 

27 

28 

32 

34 
36 
36 

37 
41 
41 
42 

42 

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51 
51 
55 
57 
58 
61 
61 
63 
64 
68 
69 

70 
74 
78 
80 
82 
83 


CONTENTS. 


IX 


The  nietadiscoidal  placenta 

The  zonary  placenta . 

The  diffuse  placenta . 

The  polycotyledonary  placenta  . 

Tabulation  of  placentation 
Microscopic  structure  of  the  placenta 

Illustrations      .... 

Evolution ..... 

Summary  ..... 
The  Development  of  the  Embryo  Itself 

Germ-layers       ...... 

Origin  of  the  vascular  system    . 

The  growth  of  the  embryo 
Development  of  the  Vascular  System  in  Vertebrates 

The  later  stages  of  the  foetal  circulation     . 
Development  of  the  Urogenital  System 
The  Physiological  Aspects  of  Development 
Ovulation 

Oestrum    . 
The  nutrition  of  the  ovum 

The  foetal  circulation 
Periods  of  Gestation     . 
Parturition   . 
Changes  in  the  circulation  after  birth    . 
Coitus  .         .  • 

Organic  Evolution  reconsidered 
The  Chemical  Constitution  of  the  Animal  Body 

Proximate  principles 

General  characters  of  protcids 
Certain  non-crystalline  bodies 

The  fats 

Pecilliar  fats  ....•• 

Carbohydrates       ...-•• 

Nitrogenous  metabolites    .... 

Non-nitrogenous  metabolites 
Physiological  Research  and  Physiological  Reason 
The  Blood  

Comparative      .... 

Corpuscles  .... 

History  of  the  blood-eells 

Chemical  composition  of  the  blood 

Composition  of  serum 

Composition  of  the  corpuscles    . 

The  quantity  and  distribution  of  the  blood 


page 
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89 
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91,  92 
93 
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102 
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129 
129 
137 
142 
144 
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148 
154 
154 
155 
158 
160 
161 
162 
163 


COMPARATIVE   PHYSIOLOGY. 


PAGE 

The  coagulation  of  the  blood „         .163 

Clinical  and  pathological 16*7 

Summary 169 

The  Contractile  Tissues     .........     171 

General 171 

Comparative      ..........     172 

Ciliary  movements 173 

The  irritability  of  muscle  and  nerve 175 

The  Graphic  Method  and  the  Study  of  Muscle  Physiology       .         .176 

Chronographs  and  various  kinds  of  apparatus    ....     176 

The  apparatus  used  for  the  stimulation  of  muscle .         .         .         .     179 

A  single  muscular  contraction        .         .  .         .         .         .185 

Tetanic  contraction 187 

The  muscle-tone         .         .         .         .  •       .         .         .         .         .189 

The  changes  in  a  muscle  during  contraction 189 

The  elasticity  of  muscle 189 

The  electrical  phenomena  of  muscle 191 

Chemical  changes  in  muscle 192 

Thermal  changes  in  the  contracting  muscle 195 

The  physiology  of  nerve 196 

Electrotonus 196 

Pathological  and  clinical 197 

Electrical  organs 197 

Muscular  work 198 

Circumstances  influencing  the  character  of  musular  and  nervous  ac- 
tivity     199 

The  influence  of  blood-supply  and  fatigue 199 

Separation  of  muscle  from  the  central  nervous  system        .         .     201 

The  influence  of  temperature 201 

Unstriped  muscle 202 

General 202 

Comparative      .         .         . 202 

Special  considerations       .........     203 

Functional  variations  ........     204 

Summary  of  the  physiology  of  muscle  and  nerve        .         .         .     205 
The  Nervous  System — General  Considerations        ....     208 

Experimental     .         .         .         .         .         .         .         .         .         .210 

Automatism       .         .         .         .         .         .         .         .         .         .211 

Conclusions 212 

Nervous  inhibition     .         .         .         .         .         .         .         .         .212 

The  Circulation  oe  the  Blood 214 

General 214 

The  mammalian  heart        ........     215 

Circulation  in  the  mammal 219 


CONTENTS. 


XI 


The  action  of  the  mammalian  heart  . 

The  velocity  of  the  blood  and  blood-pressure 

General     ..... 

Comparative      .... 
The  circulation  under  the  microscope 
The  characters  of  the  blood-flow 
Blood-pressure.         .... 

The  Heart 

The  cardiac  movements    . 

The  impulse  of  the  heart . 

Investigation  of  the  heart-beat  from  within 

The  cardiac  sounds  .... 

Causes  of  the  sounds 
Endo-cardiac  pressures 
The  work  of  the  heart 
Variations  in  the  cardiac  pulsation  . 

Comparative      .... 
The  pulse 

Features  of  an  arterial  pulse-tracing 

Venous  pulse     .... 

Pathological      .... 

Comparative      .... 
The  beat  of  the  heart  and  its  modifications 
The  nervous  system  in  relation  to  the  heart 
Influence  of  the  vagus  nerve  on  the  heart 

Conclusions       ..... 
The  accelerator  nerves  of  the  heart  . 
The  heart  in  relation  to  blood-pressure 

The  influence  of  the  quantity  of  blood 
The  capillaries 
Special  considerations 

Pathological 

Personal  observations 

Comparative 

Evolution  . 

Summary  of  the  physiology  of  the  circulat 
Digestion  of  Food 

Foodstuffs,  milk,  etc 

Embryological  . 

Comparative 
Structure,  arrangement,  and  significance  of  the 
The  digestive  juices . 

Saliva  and  its  action 

Secretion  of  the  different  elands 


teeth 


PAGE 

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566 
267 
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269 
274 
274 
280 
286 
286 
297 
297 
298 


Xll 


COMPARATIVE   PHYSIOLOGY. 


Comparative 

Gastric  juice 

Bile  . 

General 

Pigments  . 

Digestive  action 

Comparative 

Pancreatic  secretion 

Succus  entericus 

Comparative 
Secretion  as  a  physiological  process  . 

Secretion  of  the  salivary  glands 

Secretion  by  the  stomach  . 

The  secretion  of  bile  and  pancreatic 
The  nature  of  the  act  of  secretion     . 

Self-digestion  of  the  digestive  organs 

Comparative      .... 
The  alimentary  canal  of  the  vertebrate 
The  movements  of  the  digestive  organs 

Deglutition 

Comparative 

The  movements  of  the  stomach 

Comparative 

Pathological 

The  intestinal  movements . 

Defecation 

Vomiting  .... 

Comparative 
The  removal  of  digestive  products  f 

Lymph  and  chyle 

The  movements  of  the  lymph 

Pathological 

Faeces         .     ■»    . 

Pathological 
The  changes  produced  in  the  food  in 

General 

Comparative 

Pathological 
Special  considerations 

Various 

Evolution  . 

Summary  . 
The  Respiratory  System 

General 


juice 


comparative 


om  the  alimentary  canal 


the  alimentary 


canal 


PAGE 

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299 
301 
301 
302 
303 
304 
305 
307 
310 
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315 
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323 
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331 
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354 
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357 
359 
359 
359 
363 
364 
366 
366 


CONTENTS, 


xin 


blood 


espiration 


Anatomical         .... 
The  entrance  and  exit  of  air 

The  muscles  of  respiration 

Types  of  respiration  . 
Comparative      ..... 

Personal  observation 
The  quantity  of  air  respired 
The  respiratory  rhythm     . 

General     ..... 

Pathological      .         . 

Respiratory  sounds    . 
Comparison  of  the  inspired  and  the  expired  air 
Respiration  in  the  blood    . 
Haemoglobin  and  its  derivatives 

General 

Blood-spectra     .... 

Comparative      .... 

The  nitrogen  and  the  carbon  dioxide  of  the 

Foreign  gases  and  respiration    . 
Respiration  in  the  tissues .... 
The  nervous  system  in  relation  to  respiration 
The  influence  of  the  condition  of  the  blood  on  i 
The  influence  of  respiration  on  the  circulation 

General     ...... 

Comparative      ..... 

The  respiration  and  circulation  in  asphyxia 

Pathological       ..... 
Peculiar  respiratory  movements 

Coughing,  laughing,  etc.     . 

Comparative      ..... 
Special  considerations        .... 

Pathological  and  clinical    . 

Personal  observation 

Evolution  ...... 

Summary  of  the  physiology  of  respiration 
Protective  and  Excretorv  Functions  of  the 

General     ...... 

Comparative 
The  excretory  function  of  the  skin    . 

Normal  sweat    ..... 

Pathological      ...... 

Comparative — Respiration  by  the  skin 

Death  from  suppression  of  the  functions  of  the 
The  excretion  of  perspiration    .... 


Skin 


skin 


PAGE 

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412 


xiv  COMPARATIVE   PHYSIOLOGY. 

PAGE 

Experimental 413 

Absorption  by  the  skin 413 

Comparative      ..........  413 

Summary  ...........  413 

Excretion  by  the  Kidney -       .        .        .         .  415 

Anatomical         ..........  415 

Comparative      .                  415 

Urine  considered  physically  and  chemically  .  .  .  .  .419 
Specific  gravity .         .         .         .         .         .         .         .         .         .419 

Color .419 

Eeaction 419 

Quantity    .         .         .         .         .         .         .         .         .         .         .  419 

Composition :  Nitrogenous  crystalline  bodies      ....  420 

Non-nitrogenous  organic  bodies          ......  420 

Inorganic  salts .         .         .         .         .         .         .         .         ,  420 

Abnormal  urine  .         .         .         .         .         .         .         .         .421 

Comparative      .         .         .         .         .         .         .         ...         .  421 

The  secretion  of  urine 421 

Theories  of  secretion          .         .         .         .  '       .         .         .         .  421 

Nervous  influence       .         .         .         .         .         .         .         .  423 

Pathological 423 

The  expulsion  of  urine      .........  424 

General .424 

Facts  of  experiment  and  of  experience       .....  424 

Pathological '  .         .426 

Summary  of  urine  and  the  functions  of  the  kidneys  ....  426 

Comparative       ..........  426 

The  Metabolism  of  the  Body 428 

General  remarks         .........  428 

The  metabolism  of  the  liver      ........  428 

The  glycogenic  function 429 

The  uses  of  glycogen          ........  429 

Metabolism  of  the  spleen 429 

Histological 430 

Chemical 430 

The  construction  of  fat 432 

General  and  experimental           .......  432 

Histological       .         . 433 

Changes  in  the  cells  of  the  mammary  gland       ....  434 

Nature  of  fat  formation              434 

Milk  and  colostrum  .........  435 

Pathological 435 

Comparative      ..........  436 

The  study  of  tlie  metabolic  processes  by  other  methods     .         .         .  436 


CONTENTS. 


xv 


PAGE 

Various  tabular  statements        .......  437 

Starvation  and  its  lessons           .......  537 

Comparative 438 

Diets 439 

Feeding  experiments         .........  439 

General 439 

Nitrogeneous  equilibrium  ........  440 

Comparative 441 

The  effects  of  gelatine  in  the  diet 441 

Fat  and  carbohydrates 442 

Comparative      ..........  442 

The  effects  of  salts,  water,  etc.,  on  the  diet         ....  443 

Patholgical 443 

Water 443 

The  energy  of  the  animal  body          I         .....  443 

Tabular  statements   .........  444 

Animal  heat 445 

General     ...........  445 

Comparative      ..........  445 

The  regulation  of  temperature 447 

Cold-blooded  and  warm-blooded  animals  compared     .         .         .  448 

Theories  of  heat  formation  and  heat  regulation ....  448 

Pathological .         .          .  449 

Special  considerations       .........  450 

Evolution 450 

Hibernation       ..........  451 

Daily  variations  in  temperature  in  man  and  other  mammals       .  452 

The  influence  of  the  nervous  system  on  metabolism  (nutrition).         .  452 

Experimental  facts °  452 

Discussion  of  their  significance 455 

General  considerations,  clinical  and  pathological        .         .         .456 

Summary  of  metabolism    ........  458 

The  Spinal  Cord — General 461 

General 461 

Anatomical        ..........  461 

The  reflex  functions  of  the  spinal  cord 466 

General  and  experimental ........  466 

Evolution  and  heredity 468 

Inhibition  of  reflexes 469 

The  spinal  cord  as  a  conductor  of  impulses 469 

Anatomical 469 

Decussation        ..........  471 

Pathological 471 

Paths  of  impulses 473 


XVI 


COMPARATIVE   PHYSIOLOGY. 


ain  with  another 


ng  function  ? 


The  automatic  functions  of  the  spinal  cord 

General  . 

Spinal  phenomena 
Special  considerations 

Comparative 

Evolution 

Synoptical 
The  Bkain  .... 

General  and  anatomical 
Animals  deprived  of  their  cerebrum 

Behavior  of  various  animals  and  its  significance 
Have  the  semicircular  canals  a  co-ordinat 

Discussion  of  the  phenomena     . 
Forced  movements 
Functions  of  the  cerebral  convolutions 

Comparative      .... 

Individual  differences  in  brains 

The  connection  of  one  part  of  the  br 

The  cerebral  cortex  . 

Theories  of  different  observers 

The  circulation  in  the  brain 

Sleep — hibernation — dreaming 

Hypnotism,  etc. 

Illustrations  of  localization 
Functions  of  other  portions  of  the  brain 

The  corpus  striatum  and  the  optic  thalamus 

Corpora  quadrigemina 

The  cerebellum 

Pathological 

Crura  cerebri  and  pons  Varolii 

Pathological 

Medulla  oblongata     . 

Special  considerations 

Embryological   . 

Evolution  .... 

Synoptical 
General  Remarks  on  the  Senses 

Anatomical 

General  principles 
The  Skin  as  an  Organ  ok  Sense 

General     .... 
Pressure  sensations  . 
Thermal  sensations  . 
Tactile  sensibility 


CONTENTS.  xvii 

PAGE 

The  muscular  sense 524 

General      ...........  524 

Pathological 524 

Comparative      .         .         .         .         .         .         .         .         .         .524 

Synoptical          ..........  525 

Vision 526 

Anatomical 527 

Einbryological  .         .         .         .         .         .         .         .         .         .528 

Dioptrics  of  vision    ..........  531 

Accommodation  of  the  eye 532 

Alterations  in  the  size  of  the  pupil 533 

Phenomena  and  their  explanations     ......  534 

Pathological 536 

Comparative      .......                   .         .  536 

Optical  imperfections  of  the  eye 536 

Anomalies  of  refraction 536 

Visual  sensations 538 

General 538 

Affections  of  the  retina      ........  540 

The  laws  of  retinal  stimulation  .......  541 

The  visual  angle 542 

Color- vision , 543 

Psychological  aspects  of  vision          .......  543 

The  visual  field 543 

After-images,  etc.      .........  543 

Co-ordination  of  the  two  eyes  in  vision      ......  544 

The  visual  axes          .         .         .         .         .         .         .         .         .  544 

Ocular  movements     .........  544 

Estimation  of  the  size  and  distance  of  objects    ....  548 

Solidity 548 

Protective  mechanisms  of  the  eye      .......  549 

Special  considerations 551 

Comparative      .         .         .         .         .         .         .         .         .         .551 

Evolution 552 

Pathological 554 

Brief  synopsis  of  the  physiology  of  vision  .....  554 

Hearing 557 

General     ...........  557 

Anatomical         ..........  558 

The  membrana  tympani     ........  558 

The  auditory  ossicles          ........  559 

Muscles  of  the  middle  ear 561 

The  Eustachian  tube 562 

Pathological 563 

B 


XY1U 


COMPARATIVE   PHYSIOLOGY. 


PAGE 

Auditory  sensations,  perceptions,  judgments 

.     567 

.     567 

.     568 

Synopsis  of  the  physiology  of  hearing 

.     672 

The  Senses  of  Smell  and  Taste 

.     573 

Smell 

.     573 

Anatomical        ..... 

.     573 

.     573 

.     574 

,     575 

,     576 

.     576 

Experimental    ...... 

.     577 

.     578 

The  Cerebro-Spinal  System  of  Nerves 

.     579 

.     579 

.     5  SO 

Additional  experiments 

.     580 

.     580 

.     580 

.     580 

The  motor-oculi,  or  third  nerve . 

.     581 

The  trochlear,  or  fourth  nerve  . 

.     582 

The  abducens,  or  sixth  nerve 

.     282 

The  trigeminus,  or  fifth  nerve    . 

.     583 

The  glossopharyngeal,  or  ninth  nerve 

.     585 

The  pneumogastric,  or  tenth  nerve    . 

.     586 

The  spinal  accessory,  or  eleventh  nerve 

.     587 

The  hypoglossal,  or  twelfth  nerve 

.     588 

Relations  of  the  cerebrospinal  and  sympatheti 

c  syst 

ems 

.     588 

Recent  views  on  this  subject 

.     588 

The  Voice 

.     592 

Physical    ...... 

.     592 

Anatomical        ..... 

.     593 

Voice-formation        .... 

.     593 

.     598 

Oomparative      ..... 

.     598 

.     600 

CONTENTS. 


XIX 


Evolution  . 

Summary  . 
Certain  Tissues  . 

Connective  tissue 

Elastic  tissue     . 

Bone 

Cartilage  . 
Locomotion  . 

Anatomical 

Mechanical 

Standing    . 

Walking   . 

Running    . 

Jumping   . 

Hopping    . 

Comparative :  the  gait  of 

The  foot  of  the  horse 

Different  gaits :  general 

Walking   . 

The  amble 

The  trot    . 

The  gallop 

Evolution  . 


juadruped 


PAGE 
600 

601 
602 
003 
604 
604 
604 
610 
610 
610 
610 
611 
613 
614 
614 
614 
616 
621 
623 
624 
624 
624 
627 


COMPARATIVE   PHYSIOLOGY. 


GENERAL   BIOLOGY. 
INTRODUCTION. 

Biology  (fiios,  life  ;  Aoyos,  a  dissertation)  is  the  science  which. 
treats  of  the  nature  of  living  things  ;  and,  since  the  properties 
of  plants  and  animals  can  not  be  explained  without  some  knowl- 
edge of  their  form,  this  science  includes  morphology  (jioptfir), 
form  ;  Aoyo?,  a  dissertation)  as  well  as  physiology  (<]>v<ns,  na- 
ture ;  \oyos). 

Morphology  describes  the  various  forms  of  living  things  and 
their  parts  ;  physiology,  their  action  or  function. 

General  biology  treats  neither  of  animals  nor  plants  exclu- 
sively. Its  province  is  neither  zoology  nor  botany  ;  but  it  at- 
tempts to  define  wbat  is  common  to  all  living  things.  Its  aim 
is  to  determine  the  properties  of  organic  beings  as  such,  rather 
than  to  classify  or  to  give  an  exhaustive  account  of  either  ani- 
mals or  plants.  Manifestly,  before  this  can  be  done,  living 
things,  both  animal  and  vegetable,  must  be  carefully  compared, 
otherwise  it  would  be  impossible  to  recognize  differences  and 
resemblances  ;  in  other  words,  to  ascertain  what  they  have  in 
common . 

When  only  the  highest  animals  and  plants  are  contem- 
plated, the  differences  between  them  seem  so  vast  that  they 
appear  to  have,  at  first  sigbt,  nothing  in  common  but  that  they 
are  living  :  between  a  tree  and  a  dog  an  infant  can  discrimi- 
nate ;  but  there  are  microscopic  forms  of  life  that  tbus  far  defy 
the  most  learned  to  say  whether  they  belong  to  the  animal  or 
the  vegetable  world.  As  we  descend  in  the  organic  series,  the 
lines  of  distinction  g-row  fainter,  till  they  seem  finally  to  all  but 
disappear. 


2  COMPARATIVE   PHYSIOLOGY. 

But  let  ns  first  inquire  :  What  are  the  determining  charac- 
teristics of  living  things  as  such  ?  By  what  barriers  are  the 
animate  and  inanimate  worlds  separated  ?  To  decide  this,  falls 
within  the  province  of  general  biology. 

Living  things  grow  by  interstitial  additions  of  particles  of 
matter  derived  from  without  and  transformed  into  their  own 
substance,  while  inanimate  bodies  increase  in  size  by  superficial 
additions  of  matter  over  which  they  have  no  power  of  decompo- 
sition and  recomposition  so  as  to  make  them  like  themselves. 
Among  lifeless  objects,  crystals  approach  nearest  to  living 
forms  ;  but  the  crystal  builds  itself  up  only  from  material  in 
solution  of  the  same  chemical  composition  as  itself. 

The  chemical  constitution  of  living  objects  is  peculiar.  Car- 
bon, hydrogen,  oxygen,  and  nitrogen  are  combined  into  a  very 
complex  whole  or  molecule,  as  protein  ;  and,  when  in  combina- 
tion with  a  large  proportion  of  water,  constitute  the  basis  of  all 
life,  animal  and  vegetable,  known  as  protoplasm.  Only  living 
things  can  manufacture  this  substance,  or  even  protein. 

Again,  in  the  very  nature  of  the  case,  protoplasm  is  continu- 
ally wasting  by  a  process  of  oxidation,  and  being  built  up  from 
simpler  chemical  forms.  Carbon  dioxide  is  an  invariable  prod- 
uct of  this  waste  and  oxidation,  while  the  rest  of  the  carbon, 
the  hydrogen,  oxygen,  and  nitrogen  are  given  back  to  the  in- 
organic kingdom  in  simpler  forms  of  combination  than  those 
in  which  they  exist  in  living  beings.  It  will  thus  be  evident 
that,  while  the  flame  of  life  continues  to  burn,  there  is  constant 
chemical  and  physical  change.  Matter  is  being  continuously 
taken  from  the  world  of  things  that  are  without  life,  trans- 
formed into  living  beings,  and  then  after  a  brief  existence  in 
that  form  returned  to  the  source  from  which  it  was  originally 
derived.  It  is  true,  all  animals  require  their  food  in  organized 
form — that  is,  they  either  feed  on  animal  or  plant  forms  ;  but 
the  latter  derive  their  nourishment  from  the  soil  and  the  atmos- 
phere, so  that  the  above  statement  is  a  scientific  truth. 

Another  highly  characteristic  pi'operty  of  all  living  things 
is  to  be  sought  in  their  periodic  changes  and  very  limited  dura 
tion.  Every  animal  and  plant,  no  matter  what  its  rank  in  the 
scale  of  existence,  begins  in  a  simple  form,  passes  through  a 
series  of  changes  of  varying  degrees  of  complexity,  and  finally 
declines  and  dies  ;  which  simply  means  that  it  rejoins  the  in- 
animate kingdom  :  it  passes  into  another  world  to  which  it 
formerly  belonged. 


GENERAL   BIOLOGY.  3 

Living  things  alone  give  rise  to  living  things  ;  protoplasm 
alone  can  heget  protoplasm  ;  cell  begets  cell.  Omne  animal 
(anima,  life)  ex  ovo  applies  with  a  wide  interpretation  to  all 
living  forms. 

From  what  has  been  said  it  will  appear  that  life  is  a  condi- 
tion of  ceaseless  change.  Many  of  the  movements  of  the  pro- 
toplasm composing  the  cell-units  of  which  living  beings  are 
made  are  visible  under  the  microscope;  their  united  effects  are 
open  to  common  observation — as,  for  example,  in  the  move- 
ments of  animals  giving  rise  to  locomotion  we  have  the  joint 
result  of  the  movements  of  the  protoplasm  composing  millions 
of  muscle-cells.  But,  beyond  the  powers  of  any  microscope  that 
has  been  or  probably  ever  will  be  invented,  there  are  molecular 
movements,  ceaseless  as  the  flow  of  time  itself.  All  the  pro- 
cesses which  make  up  the  life-history  of  organisms  involve  this 
molecular  motion.  The  ebb  and  flow  of  the  tide  may  symbolize 
the  influx  and  efflux  of  the  things  that  belong  to  the  inanimate 
world,  into  and  out  of  the  things  that  live. 

It  follows  from  this  essential  instability  in  living  forms  that 
life  must  involve  a  constant  struggle  against  forces  that  tend 
to  destroy  it;  at  best  this  contest  is  maintained  successfully  for 
but  a  few  years  in  all  the  highest  grades  of  being.  So  long  as 
a  certain  equilibrium  can  be  maintained,  so  long  may  life  con- 
tinue and  no  longer. 

The  truths  stated  above  will  be  illustrated  in  the  simpler 
forms  of  plants  and  animals  in  the  ensuing  pages,  and  will 
become  clearer  as  each  chapter  of  this  work  is  perused.  They 
form  the  fundamental  laws  of  general  biology,  and  may  be  for- 
mulated as  follows: 

1.  Living  matter  or  protoplasm  is  characterized  by  its  chemi- 
cal composition,  being  made  up  of  carbon,  hydrogen,  oxygen, 
and  nitrogen,  arranged  into  a  very  complex  molecule. 

2.  Its  universal  and  constant  waste  and  its  repair  by  inter- 
stitial formation  of  new  matter  similar  to  the  old. 

3.  Its  power  to  give  rise  to  new  forms  similar  to  the  parent 
ones  by  a  process  of  division. 

4.  Its  manifestation  of  periodic  changes  constituting  devel- 
opment, decay,  and  death. 

Though  there  is  little  in  relation  to  living  beings  which 
may  not  be  appropriately  set  down  under  zoology  or  botany,  it 
tends  to  breadth  to  have  a  science  of  general  biologjr  which 
deals  with  the  properties  of  things  simply  as  living,  irrespective 


COMPARATIVE  PHYSIOLOGY. 


Biol- 
ogy- 

The 
science 
of  liv- 
ing    i 
things ; 
i.  e.,of 
matter 
in  the 
living 
state. 


f  Mor- 
phol- 
ogy. 

The 
science 

of 
form, 
struct- 
ure, 
etc. 
Essen- 
tially 
statical. 


Physi- 
ology 

The 

science 
of 

action 

or 
func- 
tion. 

Essen- 
tially 

dynam- 
ical. 


Anatomy. 
The  science  of  structure; 
the  termbeiug  usually 
applied  to  the  coarser 
and  more  obvious 
composition  of  plants 
or  animals. 

Histology. 

Microscopical  anatomy. 
The  ultimate  optical 
analysis  of  structure 
by  the  aid  of  the  mi- 
croscope ;  separated 
from  anatomy  only  as 
a  matter  of  conven- 
ience. 

Taxonomy. 

The  classification  of  liv- 
ing things,  based 
chiefly  on  phenomena 
of  structure. 

Distribution. 

Considers  the  position 
of  living  things  in 
space  and  time ;  their 
distribution  over  the 
present  face  of  ■  the 
earth;  and  their  dis- 
tribution and  succes- 
sion at  former  pe- 
riods, as  displayed  in 

i  fossil  remains. 

Embryology. 
The  science  of  develop- 
ment from  the  germ ; 
includes  many  mixed 
problems  pertaining 
both  to  morphology 
and  physiology.  At 
present  largely  mor- 
phological. 

Physiology. 
The  special  science  of 
the  functions  of  the 
individual  in  health 
and  in  disease ;  hence 
including  Pathology. 

Psychology. 
The  science  of  mental 
phenomena. 

Sociology. 
The  science  of  social 
life,  i.  e.,  the  life  of 
communities,  wheth- 
er of  men  or  of  lower 
animals. 


Botany. 

The 
science 
of  veg- 
etal 
living 
matter 

or 
plants. 


1 


Biol- 
ogy- 

The 
science 
of  liv- 

things ; 

i.  e.,  of 

matter 

in  the 

living 

state. 


Zool- 
ogy. 

The 
science 

of 
animal 
living 
matter 
or  ani 

mals.    J 


GENERAL  BIOLOGY.  5 

very  much  as  to  whether  they  belong  to  the  realm  of  animals  or 
plants.  The  relation  of  the  sciences  which  may  be  regarded 
as  subdivisions  of  general  biology  is  well  shown  in  the  accom- 
panying table.  * 

THE    CELL,  f 

All  living  things,  great  and  small,  are  composed  of  cells. 
Animals  may  be  divided  into  those  consisting  of  a  single  cell 
(Protozoa),  and  those  made  up  of  a  multitude  of  cells  (Metazoa) ; 
but  in  every  case  the  animal  begins  as  a  single  cell  or  ovum 
from  which  all  the  other  cells,  however  different  finally  from 
one  another  either  in  form  or  function,  are  derived  by  processes 
of  growth  and  division ;  and,  as  will  be  seen  later,  the  whole 
organism  is  at  one  period  made  up  of  cells  practically  alike  in 
structure  and  behavior.  The  history  of  each  individual  animal 
or  plant  is  the  resultant  of  the  conjoint  histories  of  each  of  its 
cells,  as  that  of  a  nation  is,  when  complete,  the  story  of  the  total 
outcome  of  the  lives  of  the  individuals  composing  it. 

It  becomes,  therefore,  highly  important  that  a  clear  notion 
of  the  characters  of  the  cell  be  obtained  at  the  outset ;  and 
this  chapter  will  be  devoted  to  presenting  a  general  account  of 
the  cell. 

The  cell,  whether  animal  or  vegetable,  in  its  most  complete 
form  consists  of  a  mass  of  viscid,  semifluid,  transparent  sub- 
stance (protoplasm),  a  cell  wall,  and  a  more  or  less  circular 
body  (nucleus)  situated  generally  centrally  within;  in  which, 
again,  is  found  a  similar  structure  (nucleolus) . 

This  description  applies  to  both  the  vegetable  and  the  ani- 
mal cell ;  but  the  student  will  find  that  the  greater  proportion 
of  animal  cells  have  no  cell  wall,  and  that  very  few  vegetable 
cells  are  without  it.  But  there  is  this  great  difference  between 
the  animal  and  vegetable  cell:  the  former  never  has  a  cellulose 
wall,  while  the  latter  rarely  lacks  such  a  covering.  In  every 
case  the  cell  wall,  whether  in  animal  or  vegetable  cells,  is  of 
greater  consistence  than  the  rest  of  the  cell.  This  is  especially 
true  of  the  vegetable  cell. 

It  is  doubtful  whether  there  are  any  cells  without  a  nucleus, 
while  not  a  few,  especially  when  young  and  most  active,  pos- 

*  Taken  from  the  General  Biology  of  Sedgwick  and  Wilson. 
f  The  illustrations  of  the  sections  following  will  enable  the  student  to 
form  a  generalized  mental  picture  of  the  cell  in  all  its  parts. 


6 


COMPARATIVE  PHYSIOLOGY. 


sess  several.  The  circular  form  may  be  regarded  as  the  typical 
form  of  both  cells  and  nuclei,  and  their  infinite  variety  in  size 
and  form  may  be  considered  as  in  great  part  the  result  of  the 
action  of  mechanical  forces,  such  as  mutual  pressure ;  this  is,  of 
course,  more  especially  true  of  shape.  Reduced  to  its  greatest 
simplicity,  then,  the  cell  may  be  simply  a'  mass  of  protoplasm 
with  a  nucleus. 

It  seems  probable  that  the  numerous  researches  of  recent 
years  and  others  now  in  progress  will  open  up  a  new  world  of 


Fig.  1.— Nuclear  division.  A-II,  karyokinesis  of  a  tissue  cell.  A,  nuclear  reticulum 
in  its  ordinary  state.  B,  preparing  for  division  ;  the  contour  is  less  defined,  and 
the  fibers  thicker  and  less  intricate.  C,  wreath-stage  ;  the  chromatin  is  arranged 
in  a  complicated  looping  round  the  equator  of  the  achromatin  spindle.  D,  nio- 
naster-stage  ;  the  chromatin  now  appears  as  centripetal  equatorial  V's,  each  of 
which  should  be  represented  as  double.  B,  a  migration  of  the  half  of  each  chro- 
matin loop  towards  opposite  poles  of  the  spindle.  F,  diaster-stage  ;  the  chroma- 
tin forme  a  star,  round  each  pole  of  a  spindle,  each  aster  being  connected  by 
Strands  of  achromatin.  G,  daughter-wreath  stage;  the  newly  formed  nuclei  are 
passing  through  their  retrogressive  development,  which  is  completed  in  the  rest- 
ing stage,  H.  d-f,  karyokinesis  of  an  egg-cell,  showing  the  smaller  amount  of 
chromatin  than  'in  the  tissue-cell.  The  stages  d,  e,fi  correspond  to  D,  E,  F,  re- 
spectively. The  polar  star  at  the  end  of  the  spindle  is  composed  of  protoplasm- 
granules  of  the  cell  itself,  and  must  not  be  mistaken  for  the  diaster(F).  The 
coarse  lii  es  represent  the  chromatin,  the  fine  lines  the  achromatin,  and  the  dotted 
lines  cell-gran  tiles.  (Chiefly  modified  from  (Hemming.)  X-Z,  direct  nuclear  divis- 
ion in  the  cells  of  the  embryonic  integument  of  the  European  scorpion.  After 
Blochmann  (Haddorl). 

cell  biology  which  will  greatly  advance  our  knowledge,  espe- 
cially in  the  direction  of  increased  depth  and  accuracy. 


GENERAL  BIOLOGY.  7 

Though  many  points  are  still  in  dispute,  it  may  be  safely 
said  that  the  nucleus  plays,  in  most  cells,  a  role  of  the  highest 
importance;  in  fact,  it  seems  as  though  we  might  regard  the 
nucleus  as  the  directive  brain,  so  to  speak,  of  the  individual 
cell.  It  frequently  happens  that  the  behavior  of  the  body  of 
the  cell  is  foreshadowed  by  that  of  the  nucleus.  Thus  fre- 
quently, if  not  always,  division  of  the  body  of  the  nucleus  pre- 
cedes that  of  the  cell  itself,  and  is  of  a  most  complicated  char- 
acter (karyokinesis  or  mitosis).  The  cell  wall  is  of  subordinate 
importance  in  the  processes  of  life,  though  of  great  value  as  a 
mechanical  support  to  the  protoplasm  of  the  cell  and  the  aggre- 
gations of  cells  known  as  tissues.  The  greater  part  of  a  tree 
may  be  said  to  be  made  up  of  the  thickened  walls  of  the  cells, 
and  these  are  destitute  of  true  vitality,  unless  of  the  lowest 
order ;  while  the  really  active,  growing  part  of  an  old  and  large 
tree  constitutes  but  a  small  and  limited  zone,  as  may  be  learned 
from  the  plates  of  a  work  on  modern  botany  representing  sec- 
tions of  the  wood. 

Animals,  too,  have  their  rigid  parts,  in  the  adult  state  espe- 
cially, resulting  from  the  thickening  of  a  part  of  the  whole  of 
the  cell  by  a  deposition  usually  of  salts  of  lime,  as  in  the  case  of 
the  bones  of  animals.  But  in  some  cases,  as  in  cartilage,  the 
cell  wall  or  capsule  undergoes  thickening  and  consolidation, 
and  several  may  fuse  together,  constituting  a  matrix,  which  is 
also  made  up  in  part,  possibly,  of  a  secretion  from  the  cell  pro- 
toplasm. In  the  outer  parts  of  the  body  of  animals  we  have  a 
great  abundance  of  examples  of  thickening  and  hardening  of 
cells.  Very  well-known  instances  are  the  indurated  patches 
of  skin  {epithelium)  on  the  palms  of  the  hands  and  else- 
where. 

It  will  be  scarcely  necessary  to  remark  that  in  cells  thus 
altered  the  mechanical  has  largely  taken  the  place  of  the  vital 
in  function.  This  at  once  harmonizes  with  and  explains  what  is 
a  matter  of  common  observation,  that  old  animals  are  less  act- 
ive— have  less  of  life  within  them,  in  a  word,  than  the  young. 
Chemically,  the  cellulose  wall  of  plant-cells  consists  of  carbon, 
hydrogen,  and  oxygen,  in  the  same  relative  proportion  as  exists 
in  starch,  though  its  properties  are  very  different  from  those  of 
that  substance. 

Turning  to  cell  contents,  we  find  them  everywhere  made  up 
of  a  clear,  viscid  substance,  containing  almost  always  granules 
of  varying  but  very  minute  size,  and  differing  in  consistence 


8  COMPARATIVE  PHYSIOLOGY. 

not  only  in  different  groups  of  cells,  but  often  in  the  same  cell, 
so  that  we  can  distinguish  an  outer  portion  {ectoplasm)  and  an 
inner  more  fluid  and  more  granular  region  (endojolasm). 

The  nucleus  is  a  body  with  very  clearly  defined  outline  (in 
some  cases  limited  by  a  membrane),  through  which  an  irregular 
network  of  fibers  extends  that  •  stains  more  deeply  than  any 
other  part  of  the  whole  cell. 

Owing  to  the  fact  that  it  is  so  readily  changed  by  the  action 
of  reagents,  it  is  impossible  to  ascertain  the  exact  chemical  com- 
position of  living  protoplasm  ;  in  consequence,  we  can  only 
infer  its  chemical  structure,  etc.,  from  the  examination  of  the 
dead  substance. 

In .  general,  it  may  be  said  that  protoplasm  belongs  to  the 
class  of  bodies  known  as  proteids — that  is,  it  consists  chemically 
of  carbon,  hydrogen,  a  little  sulphur,  oxygen,  and  nitrogen,  ar- 
ranged into  a  very  complex  and  unstable  molecule.  This  very 
instability  seems  to  explain  at  once  its  adaptability  for  the  man- 
ifestation of  its  nature  as  living  matter,  and  at  the  same  time  the 
readiness  with  which  it  is  modified  by  many  circumstances,  so 
that  it  is  possible  to  understand  that  life  demands  an  incessant 
adaptation  of  internal  to  external  conditions. 

It  seems  highly  probable  that  protoplasm  is  not  a  single  pro- 
teid  substance,  but  a  mixture  of  such  ;  or  let  us  rather  say,  fur- 
nishes these  when  chemically  examined  and  therefore  dead. 

Very  frequently,  indeed  generally,  protoplasm  contains  other 
substances,  as  salts,  fat,  starch,  chlorophyl,  etc. 

From  the  fact  that  the  nucleus  stains  differently  from  the 
cell  contents,  we  may  infer  a  difference  between  them,  physi- 
cal and  especially  chemical.  It  (nucleus)  furnishes  on  analysis 
nuclein,  which  contains  the  same  elements  as  protoplasm  (with 
the  exception  of  sulphur)  together  with  phosphorus.  Nuclei 
have  great  resisting  power  to  ordinary  solvents  and  even  the 
digestive  juices. 

Inasmuch  as  all  vital  phenomena  are  associated  with  proto- 
plasm, it  has  been  termed  the  "  physical  basis  of  life  "  (Hux- 

ley)\ 

Tissues. — A  collection  of  cells  performing  a  similar  physio- 
logical action  constitutes  a  tissue. 

Generally  the  cells  are  held  together  either  by  others  with 
that  sole  function,  or  by  cement  material  secreted  by  them- 
selves. An  organ  may  consist  of  one  or  several  tissues.  Thus 
the  stomach  consists  of  muscular,  serous,  connective,  and  gland- 


GENERAL   BIOLOGY.  9 

ular  tissues,  besides  those  constituting  its  blood-vessels,  lym- 
phatics, and  nerves.  But  all  of  the  cells  of  each  tissue  have, 
speaking  generally,  the  same  function.  The  student  is  referred 
to  works  on  general  anatomy  and  histology  for  classifications 
and  descriptions  of  the  tissues.     See  also  page  603. 

The  statements  of  this  chapter  will  find  illustration  in  tbe 
pages  immediately  following,  after  which  we  shall  return  to 
the  subject  of  the  cell  afresh. 

Summary. — The  typical  cell  consists  of  a  wall,  protoplasmic 
contents,  and  a  nucleus.  The  vegetable  cell  has  a  limiting 
membrane  of  cellulose.  Cells  undergo  differentiation  and  may 
be  united  into  groups  forming  tissues  which  serve  one  or  more 
definite  purposes. 

The  chemical  constitution  of  protoplasm  is  highly  complex 
and  unstable.  The  nucleus  plays  a  prominent  part  in  the  life- 
history  of  the  cell,  and  seems  to  be  essential  to  its  perfect  devel- 
opment and  greatest  physiological  efficiency. 

UNICELLULAR   PLANTS. 

Yeast  (Torula,  Saccharomyces  Cerevisice). 

The  essential  part  of  the  common  substance,  yeast,  may  be 
studied  to  advantage,  as  it  affords  a  simple  type  of  a  vast  group 
of  organisms  of  profound  interest  to  the  student  of  physiology 
and  medicine.  To  state,  first,  the  main  facts  as  ascertained  by 
observation  and  experiment  : 

Morphological. — The  particles  of  which  yeast  is  composed 
are  cells  of  a  circular  or  oval  form,  of  an  average  diameter  of 
about  s-gVo  of  an  inch. 

Each  individual  torula  cell  consists  of  a  transparent  homo- 
geneous covering  (cellulose)  and  granular  semifluid  contents 
(protoplasm).  Within  the  latter  there  may  be  a  space  (vacu- 
ole) filled  with  more  fluid  contents. 

The  various  cells  produced  by  budding  may  remain  united 
like  strings  of  beads.  Collections  of  masses  composed  of  four 
or  more  subdivisions  (ascospores),  which  finally  separate  by  rup- 
ture of  the  original  cell  wall,  having  thus  become  themselves  in- 
dependent cells,  maybe  seen  more  rarely  (endogenous  division). 

The  yeast-cell  is  now  believed  to  possess  a  nucleus. 

Chemical. — When  yeast  is  burned  and  the  ashes  analyzed, 
they  are  found  to  consist  chiefly  of  salts  of  potassium,  calcium, 
and  magnesium. 


10 


COMPARATIVE   PHYSIOLOGY. 


The  elements  of  which  yeast  is  composed  are  C,  H,  O,  N,  S, 
P,  K,  Mg,  and  Ca;  but  chiefly  the  first  four. 

Physiological. — If  a  little  of  the  powder  obtained  by  drying 
yeast  at  a  temperature  below  blood-heat  be  added  to  a  solution 

of  sugar,  and  the  lat- 


ter be  kept  warm, 
bubbles  of  carbon  di- 
oxide will  be  evolved, 
causing  the  mixture 
to  become  frothy ;  and 
the  fluid  will  acquire 
an  alcoholic  charac- 
ter {fermentation). 

If  the  mixture  be 
raised  to  the  boiling- 
point,  the  process  de- 
scribed at  once  ceases. 

It  may  be  further 
noticed  that  in  the 
fermenting  saccha- 
rine solution  there  is 
a  gradual  increase  of 
turbidity.  All  of  these 
changes  go  on  per- 
fectly well  in  the  to- 
tal absence  of  sun- 
light. 

Yeast  -  cells  are 
found  to  grow  and 
reproduce  abundant- 
ly iu  an  artificial  food 
solution  consisting  of 
a   dilute    solution   of 

Fig.  4.— Further  development  of  the  forms  represented    certain  Salts,  together 
in  Fig.  3.  .,,  '       6 

with  sugar. 

Conclusions. — What  are  the  conclusions  which  may  be  legiti- 
mately drawn  from  the  above  facts  ? 

That  the  essential  part  of  yeast  consists  of  cells  of  about  the 
size  of  mammalian  blood-corpuscles,  but  with  a  limiting  wall 
of  a  substance  different  from  the  inclosed  contents,  which  latter 
is  composed  chiefly  of  that  substance  common  to  all  living 
things — protoplasm;  that  like  other  cells  they  reproduce  their 


Fig.  2. — Various  stages  in  the  development  of  brewer's 
yeast,  seen,  with  the  exception  of  the  first  in  the 
series,  with  an  ordinary  high  power  (Zeiss,  D.  4)  of 
the  microscope.  The  first  is  greatly  magnified 
(Gundlach's  ^  immersion  lens).  The  second  series 
of  four  represents  stages  in  the  division  of  a  single 
cell  ;  and  the  third  series  a  branching  colony. 
Everywhere  the  light  areas  indicate  vacuoles. 


Fig.  3.— The  cndogonidia  (ascospore)  phase  of  repro- 
duction—i.  e.,  endogenous  division. 


GENERAL   BIOLOGY. 


11 


kind,  and  in  this  instance  by  two  methods :  gemmation  giving 
rise  to  the  bead- like  aggregations  alluded  to  above;  and  in- 
ternal division  of  the  protoplasm  (endogenous  division). 

From  the  circumstances  under  which  growth  and  reproduc- 
tion take  place,  it  will  be  seen  that  the  original  protoplasm  of 
the  cells  may  increase  its  bulk  or  grow  when  supplied  with 
suitable  food,  which  is  not,  as  will  be  learned  later,  the  same  in 
all  respects  as  that  on  which  green  plants  thrive ;  and  that  this 
may  occur  in  darkness.  But  it  is  to  be  especially  noted  that  the 
protoplasm  resulting  from  the  action  of  the  living  cells  is 
wholly  different  from  any  of  the  substances  used  as  food.  This 
power  to  construct  protoplasm  from  inanimate  and  unorgan- 
ized materials,  reproduction,  and  fermentation  are  all  proper- 
ties characteristic  of  living  organisms  alone. 

It  will  be  further  observed  that  these  changes  all  take  place 
within  narrow  limits  of  temperature;  or,  to  put  the  matter 
more  generally,  that  the  life-history  of  this  humble  organism 
can  only  be  unfolded  under  certain  well-defined  conditions. 

Protococcus  (Protococcus  pluvialis). 

The  study  of  this  one- celled  plant  will  afford  instructive 
comparison  between  the  ordinary  green  plant  and  the  colorless 
plants  or  fungi. 


Fig.  5. 


Fig.  6. 


Fig.  7. 

Figs.  5  to  7  represent  successive  stages  observed  in  the  life-history  of  Protococci 

scraped  from  the  bark  of  a  tree. 
Fig.  5.— A  group  in  the  dried  state,  illustrating  method  of  division. 
Fig.  6.— One  of  the  above  after  two  days'  immersion  in  water. 
Fig.  7. — Various  phases  in  the  later  motile  stage  assumed  by  the  above  specimens. 

The  nucleus  is  denoted  by  nc:  the  cell  wall  by  c.w  ;  and  the  coloring-matter  by 

the  dark  spot.    On  the  left  of  Fig.  7  an  individual  may  be  seen  that  is  "devoid  of  a 

cell  wall. 


12  COMPARATIVE  PHYSIOLOGY. 

Like  Tovula  it  is  selected  because  of  its  simple  nature,  its 
abundance,  and  tbe  ease  with  which  it  may  be  obtained,  for  it 
abounds  in  water-barrels,  standing-  pools,  dri n king-troughs, 
etc. 

Morphological. — Protococcus  consists  of  a  structureless  wall 
and  viscid  granular  contents,  i.  e.,  of  cellulose  and  protoplasm. 

The  protoplasm  may  contain  starch  and  a  red  or  green  color- 
ing matter  (chlorophyl) .  It  probably  contains  a  nucleus.  The 
cell  is  mostly  globular  in  form. 

Physiological. — It  reproduces  by  division  of  the  original  cell 
(fission)  into  similar  individuals,  and  by  a  process  of  budding 
and  constriction  (gemmation)  which  is  much  rarer.  Under  the 
influence  of  sunlight  it  decomposes  carbon  dioxide  (CO2),  fixing 
the  carbon  and  setting  the  oxygen  free.  It  can  flourish  per- 
fectly in  rain-water,  which  contains  only  carbon  dioxide,  salts 
of  ammonium,  and  minute  quantities  of  other  soluble  salts  that 
may  as  dust  have  been  blown  into  it. 

There  is  a  motile  form  of  this  unicellular  plant,  and  in  this 
stage  it  moves  through  the  fluid  in  which  it  lives  by  means  of 
extensions  of  its  protoplasm  (cilia)  through  the  cell  wall  ;  or 
the  cell  wall  may  disappear  entirely.  Finally,  the  motile  form, 
withdrawing  its  cilia  and  clothing  itself  with  a  cellulose  coat, 
becomes  globular  and  passes  into  a  quiescent  state  again. 
Much  of  this  part  of  its  history  is  common  to  lowly  animal 
forms. 

Conclusions. — It  will  be  seen  that  there  is  much  in  common 
in  the  life-history  of  Torula  and  Protococcus.  By  virtue  of 
being  living  protoplasm  they  transform  unorganized  material 
into  their  own  substance  ;  and  they  grow  and  reproduce  by 
analogous  methods. 

But  there  are  sharply  denned  differences.  For  the  green 
plant  sunlight  is  essential,  in  the  presence  of  which  its  chloro- 
phyl prepares  the  atmosphere  for  animals  by  the  removal  of 
carbonic  anhydride  and  the  addition  of  oxygen,  while  for 
Torula  neither  this  gas  nor  sunlight  is  essential. 

Moreover,  the  fungus  (Torula)  demands  a  higher  kind  of 
food,  one  more  nearly  related  to  the  pabulum  of  animals  ;  and 
is  absolutely  independent  of  sunlight,  if  not  actually  injured  by 
it ;  not  to  mention  the  remarkable  process  of  fermentation. 


GENERAL  BIOLOGY.  13 

UNICELLULAR    ANIMALS. 

The  Proteus  Animalcule  (Amoeba). 

In  order  to  illustrate  animal  life  in  its  simpler  form  we 
choose  the  above-named  creature,  which  is  nearly  as  readily 
obtainable  as  Protococcus  and  often  under  the  same  circum- 
stances. 

Morphological. — Amoeba  is  a  microscopic  mass  of  transpar- 
ent protoplasm,  about  the  size  of  the  largest  of  the  colorless 
blood-corpuscles  of  cold-blooded  animals,  with  a  clearer,  more 
consistent  outer  zone  (ectosarc),  (although  without  any  proper 
cell  wall),  and  a  more  fluid,  granular  inner  part.  A  clear  space 
(contractile  vesicle,  vacuole)  makes  its  appearance  at  intervals 
in  the  ectosarc,  which  may  disappear  somewhat  suddenly.  This 
appearance  and  vanishing  have  suggested  the  term  pulsating 
or  contracting  vesicle.  Both  a  nucleus  and  nucleolus  may  be 
seen  in  Amoeba.  At  varying  short  periods  certain  parts  of  its 
body  ( pseudopodia)  are  thrust  out  and  others  withdrawn. 

Physiological. — Amoeba  can  not  live  on  such  food  as  proves 
adequate  for  either  Protococcus  or  Torula,  but  requires,  besides 
inorganic  and  unorganized  food,  also  organized  matter  in  the 
form  of  a  complex  organic  compound  known  as  protein,  which 
contains  nitrogen  in  addition  to  carbon,  hydrogen,  and  oxygen. 
In  fact,  Amoeba  can  prey  upon  both  plants  and  animals,  and 
thus  use  up  as  food  protoplasm  itself.  The  pseudopodia  serve 
the  double  purpose  of  organs  of  locomotion  and  prehension. 

This  creature  absorbs  oxygen  and  evolves  carbon  dioxide. 
Inasmuch  as  any  part  of  the  body  may  serve  for  the  admission, 
and  possibly  the  digestion,  of  food  and  the  ejection  of  the  use- 
less remains,  we  are  not  able  to  define  the  functions  of  special 
parts.  Amoeba  exercises,  however,  some  degree  of  choice  as  to 
what  it  accepts  or  rejects. 

The  movements  of  the  pseudopodia  cease  when  the  tempera- 
ture of  the  surrounding  medium  is  raised  or  lowered  beyond  a 
certain  point.  It  can,  however,  survive  in  a  quiescent  form 
greater  depression  than  elevation  of  the  temperature.  Thus,  at 
35°  C,  heat-rigor  is  induced;  at  40°  to  45°  C,  death  results  ; 
but  though  all  movement  is  arrested  at  the  freezing-point  of 
water,  recovery  ensues  if  the  temperature  be  gradually  raised. 
Its  form  is  modified  by  electric  shocks  and  chemical  agents, 
as  well  as  by  variations  in  the  temperature.  At  the  pres- 
ent time  it  is  not  possible  to  define  accurately  the  functions 


14 


COMPARATIVE   PHYSIOLOGY. 


of  the  vacuoles  found  in  any  of  the  organisms  thus  far  consid- 
ered. It  is  worthy  of  note  that  Amoeba  may  spontaneously 
assume  a  spherical  form,  secrete  a  structureless  covering,  and 


Fig.  14. 


Fig.  15. 


Fig.  10. 


Figs.  8  to  15,  represent  successive  phases  in  the  life-history  of  an  Amoeboid  organism- 
kept  under  constant  observation  for  three  days  ;  Fig.  16  a  similar  organism  en" 
cysted,  which  was  a  few  hours  later  set  free  by  the  disintegration  of  the  cyst. 
(All  the  figures  arc  drawn  under  Zeiss,  D.  3.) 

Fra.  8. — The  locomotor  phase  ;  the  ectoplasm  is  seen  protruding  to  form  a  pseudopo- 
(liuiri,  into  which  the  endoplasm  passes. 

Fig.  9. — A  stage  in  the  ingestive  phase.  A  vegetable  organism,,/)?,  is  undergoing  in- 
tussusception. 

Fig.  10.— A  portion  of  the  creature  represented  in  Fig.  9,  after  complete  ingestion  of 
the  food-particle. 

Fig.  11,  12. — Successive  stages  in  the  assimilative  and  excretory  processes.  Fig.  12 
represents  the  organism  some  twenty  hours  later  than  as  seen  in  Fig.  11.  The 
undigested  remnants  of  the  ingested  organism  are  represented  undergoing  ejec- 
tion (excretion)  atfp,  in  Fig.  12. 

Figs.  13,  14,  15,  represent  successive  stages  in  the  reproductive  process  of  the  same  in- 
dividual, observed  two  days  later.  It  will  be  noticed  (Fig.  13)  that  the  nucleus  di- 
vides first. 

In  tin:  above  figures,  vc,  denotes  the  contracting  vacuole  ;  nc,  the  nucleus  ;  ps,  pseu- 
dopodium  ;  M,  diatom  ;  fp,  food-particle. 


GENERAL   BIOLOGY.  15 

remain  in  this  condition  for  a  variable  period,  reminding  us  of 
the  similar  behavior  of  Torula. 

Amoeba  reproduces  by  fission,  in  which  the  nucleus  takes  a 
prominent  if  not  a  directive  part,  as  seems  likely  in  regard  to 
all  the  functions  of  unicellular  organisms. 

Conclusions. — It  is  evident  that  Amoeba  is,  in  much  of  its 
behavior,  closely  related  to  both  colored  arid  colorless  one-celled 
plants.  All  of  the  three  classes  of  organisms  are  composed  of 
protoplasm  ;  each  can  construct  protoplasm  out  of  that  which 
is  very  different  from  it  ;  each  builds  up  the  inanimate  inor- 
ganic world  into  itself  by  virtue  of  that  force  which  we  call 
vital,  but  which  in  its  essence  we  do  not  understand  ;  each  mul- 
tiplies by  division  of  itself,  and  all  can  only  live,  move,  and 
have  their  being  under  certain  definite  limitations.  But  even 
among  forms  of  life  so  lowly  as  those  we  have  been  consider- 
ing, the  differences  between  the  animal  and  vegetable  worlds 
appear.  Thus,  Amoeba  never  has  a  cellulose  wall,  and  can  not 
subsist  on  inorganic  food  alone.  The  cellulose  wall  is  not,  how- 
ever, invariably  present  in  plants,  though  this  is  generally  the 
case  ;  and  there  are  animals  (Ascidians)  with  a  cellulose  invest- 
ment. Such  are  very  exceptional  cases.  But  the  law  that  ani- 
mals must  have  organized  material  (protein)  as  food  is  without 
exception,  and  forms  a  broad  line  of  distinction  between  the 
animal  and  vegetable  kingdoms. 

Amoeba  will  receive  further  consideration  later  ;  in  the 
mean  time,  we  turn  to  the  study  of  forms  of  life  in  many  re- 
spects intermediate  between  plants  and  animals,  and  full  of  prac- 
tical interest  for  mankind,  on  account  of  their  relations  to  dis- 
ease, as  revealed  by  recent  investigations. 

PARASITIC   ORGANISMS. 

The  Fungi. 
Molds  (Penicillmm  glaucum  and  Mucor  mucedo). 

Closely  related  to  Torula  physiologically,  but  of  more  com- 
plex structure,  are  the  molds,  of  which  we  select  for  convenient 
study  the  common  green  mold  (Penieilliiim),  found  growing  in 
dark  and  moist  places  on  bread  and  similar  substances,  and  the 
white  mold  {Mucor),  which  grows  readily  on  manure. 

The  fungi  originate  in  spores,  which  are  essentially  like 
Torula  in  structure,  by  a  process  of  budding  and  longitudinal 
extension,  resulting  in  the  formation  of  transparent  branches 


16 


COMPARATIVE   PHYSIOLOGY. 


3«PP 


GENERAL  BIOLOGY.  17 

Pigs.  17  to  28. — In  the  following  figures,  ha,  denotes  ae>ial  hyphse;  sp,  sporangium; 

zy,  sygospore;  ex,  exosporium;  my,  mycelium;  mc,  mucilage;  cl,  columella;  en, 

endogonidia. 
Fro.  17.— Spore-bearin<*  hyphse  of  Mucor,  growing  from  horse-dung. 
Fti*.  1  .—The  same,  teased  out  with  needles  (A,  4). 

Figs.  19,  20,  21.— Successive  stages  in  the  development  of  the  sporangium. 
Fig.  22.— Isolated  spores  of  Mucor. 
Fig.  23. — Germinating  spores  of  the  same  mold. 
Fig.  24.— Successive  stages  in  the  germination  of  a  single  spore. 
Figs.  25,  26,  27.— Successive  phases  in  the  conjugative  process  of  Mucor. 
FiG.  28.— Successive  stages  observed  during  ten  hours  in  the  growth  of  a  conidiophore 

of  Penicillium  in  an  object-glass  culture  (D,  4). 

or  tubules,  filled  with  protoplasm  and  invested  by  cellulose 
walls,  across  which  transverse  partitions  are  found  at  regular 
intervals,  and  in  which  vacuoles  are  also  visible. 

The  spores,  when  growing  thus  in  a  liquid,  gives  rise  to  up- 
ward branches  (aerial  hyphce),  and  downward  branches  or  root- 
lets (submerged  hyphce).  These  multitudinous  branches  inter- 
lace in  every  direction,  forming  an  intricate  felt-work,  which 
supports  the  green  powder  (spores)  which  may  be  so  easily 
shaken  off  from  a  growing  mold.  In  certain  cases  the  aerial 
hyphae  terminate  in  tufts  of  branches,  which,  by  transverse 
division,  become  split  up  into  spores  (Conidia),  each  of  which 
is  similar  in  structure  to  a  yeast-cell. 

The  green  coloring  matter  of  the  fungi  is  not  chlorophyl. 
The  Conidia  germinate  under  the  same  conditions  as  Torula. 

Mucor  mucedo. — The  growth  and  development  of  this  mold 
may  be  studied  by  simply  inverting  a  glass  tumbler  over  some 
horse-dung  on  a  saucer,  into  which  a  very  little  water  has  been 
poured,  and  keeping  the  preparation  in  a  warm  place. 

Very  soon  whitish  filaments,  gradually  getting  stronger,  ap- 
pear, and  are  finally  topped  by  rounded  heads  or  spore-cases 
{Sporangia).  These  filaments  are  the  hyphce,  similar  in  struct- 
ure to  those  of  Penicillium.  The  spore-case  is  filled  with  a 
multitude  of  oval  bodies  (spores),  resulting  from  the  subdivision 
of  the  protoplasm,  which  are  finally  released  by  the  spore-case 
becoming  thinned  to  the  point  of  rupture.  The  development 
of  these  spores  take  place  in  substantially  the  same  manner  as 
those  of  Penicillium.  Sporangia  developing  spores  in  this  fash- 
ion by  division  of  the  protoplasm  are  termed  asci,  and  the  spores 
ascospores. 

So  long  as  nourishment  is  abundant  and  the  medium  of 
growth  fluid,  this  asexual  method  of  reproduction  is  the  only 
one ;  but,  under  other  circumstances,  a  mode  of  increase,  known 
as  conjugation,  arises.  Two  adjacent  hyphse  enlarge  at  the  ex- 
tremities into  somewhat  globular  heads,  bend  over  toward  each 
2 


18  COMPARATIVE   PHYSIOLOGY. 

other,  and,  meeting,  their  opposed  faces  hecome  thinned,  and 
the  contents  intermingle.  The  result  of  this  union  {zygospore) 
undergoes  now  certain  further  changes,  the  cellulose  coat  heing 
separated  into  two — an  outer,  darker  in  color  (exosporium), 
and  an  inner  colorless  one  (endosporium) . 

Under  favoring  circumstances  these  coats  burst,  and  a 
branch  sprouts  forth  from  which  a  vertical  tube  arises  that 
terminates  in  a  sporangium,  in  which  spores  arise,  as  before  de- 
scribed. It  will  be  apparent  that  we  have  in  Mucor  the  exem- 
plification of  what  is  known  in  biology  as  "  alternation  of  gen- 
erations ".—that  is,  there  is  an  intermediate  generation  be 
tween  the  original  form  and  that  in  which  the  original  is 
again*reached. 

Physiologically  the  molds  closely  resemble  yeast,  some  of 
them,  as  Mucor,  being  capable  of  exciting  a  fermentation. 

The  fungi  are  of  special  interest  to  the  medical  student,  be- 
cause many  forms  of  cutaneous  disease  are  directly  associated 
with  their  growth  in  the  epithelium  of  the  skin,  as,  for  exam- 
ple, common  ringworm  ;  and  their  great  vitality,  and  the  facil- 
ity with  which  their  spores  are  widely  dispersed,  explain  the 
highly  contagious  nature  of  such  diseases.  The  media  on  which 
they  flourish  (feed)  indicates  their  great  physiological  differ- 
ences in  this  particular  from  the  green  plants  proper.  They  are 
closely  related  in  not  a  few  respects  to  an  important  class  of 
vegetable  organisms,  known  as  bacteria,  to  be  considered  forth- 
with. 

The  Bacteria. 

The  bacteria  include  numberless  varieties  of  organisms  of 
extreme  minuteness,  many  of  them  visible  only  by  the  help  of 
the  most  powerful  lenses.  Then  size  has  been  estimated  at 
from  ^xiiny  to  i0^6o  of  an  inch  in  diameter. 

They  grow  mostly  in  the  longitudinal  direction,  and  repro- 
duce by  transverse  division,  forming  spores  from  which  new 
generations  arise. 

Some  of  them  have  vibratile  cilia,  while  the  cause  of  the 
movements  of  others  is  quite  unknown. 

As  in  many  other  lowly  forms  of  life,  there  is  a  quiescent 
as  well  as  an  active  stage.  In  this  stage  (zoogloea  form)  they 
are  surrounded  by  a  gelatinous  matter,  probably  secreted  by 
themselves. 

Bacteria  grow  and  reproduce  in  Pasteur's  solution,  rendering 
it  opaque,  as  well  as  in  almost  all  fluids  that  abound  in  proteid 


GENERAL  BIOLOGY. 


19 


matter.     That  such  fluids  readily  putrefy  is  owing  to  the  pres- 
ence of  bacteria,  the  vital  action  of  which  suffices  to  break  asun- 

Fig.  89. 


Fig.  32 


Fig.  29.—  Micrococcus,  very  like  a  spore,  but  usually  much  smaller. 

Fig.  30.— Bacterium. 

Fig.  31.— Bacillus.  The  central  filament  presented  this  segmented  appearance  as  the 
result  of  a  process  of  transverse  division  occurring  during  ten  minutes'  obser- 
vation. 

Fig.  32.— Spirillum;  various  forms.  The  first  two  represent  vibrio,  which  is  possibly 
only  a  stage  of  spirillum. 

Fig.  33. — A  drop  of  the  surface  scum,  showing  a  spirillum  aggregate  in  the  resting 
state. 

der  complex  chemical  compounds  and  produce  new  ones.  Some 
of  the  bacteria  require  oxygen,  as  Bacillus  anthracis,  while 
others  do  not,  as  the  organism  of  putrefaction,  Bacterium 
termo. 

Bacteria  are  not  so  sensitive  to  slight  variations  in  tempera- 
ture as  most  other  organisms.  They  can,  many  of  them,  with- 
stand freezing  and  high  temperatures.  All  bacteria  and  all 
germs  of  bacteria  are  killed  by  boiling  water,  though  the  spores 


20  COMPARATIVE  PHYSIOLOGY. 

are  much  more  resistant  than  the  mature  organisms  themselves. 
Some  spores  can  resist  a  dry  heat  of  140°  C. 

The  spores,  like  Torula  and  Protococcus,  bear  drying,  with- 
out loss  of  vitality,  for  considerable  periods. 

That  different  groups  of  bacteria  have  a  somewhat  different 
life-history  is  evident  from  tbe  fact  that  the  presence  of  one 
checks  the  other  in  the  same  fluid,  and  that  successive  swarms 
of  different  kinds  may  flourish  where  others  have  ceased  to 
live. 

That  these  organisms  are  enemies  of  the  constituent  cells  of 
the  tissues  of  the  highest  mammals  has  now  been  abundantly 
demonstrated.  That  they  interfere  with  the  normal  working 
of  the  organism  in  a  great  variety  of  ways  is  also  clear  ;  and 
certain  it  is  that  the  harm  they  do  leads  to  aberration  in  cell- 
life,  however  that  may  be  manifested.  They  rob  the  tissues  of 
their  nutriment  and  oxygen,  and  poison  them  by  the  products 
of  the  decompositions  they  produce.  But  apart  from  this,  their 
very  presence  as  foreign  agents  must  hamper  and  derange  the 
delicate  mechanism  of  cell-life. 

These  organisms  seem  to  people  the  air,  land,  and  waters 
with  invisible  hosts  far  more  numerous  than  the  forms  of  life 
we  behold.  Fortunately,  they  are  not  all  dangerous  to  the 
higher  forms  of  mammalian  life  ;  but  that  a  large  proportion 
of  the  diseases  which  afflict  both  man  and  the  domestic  animals 
are  directly  caused  by  the  presence  of  such  forms  of  life,  in  the 
sense  of  being  invariably  associated  with  them,  is  now  beyond 
doubt. 

The  facts  stated  above  explain  why  that  should  be  so  ;  why 
certain  maladies  should  be  infectious  ;  how  the  germs  of  dis 
ease  may  be  transported  to  a  friend  wrapped  up  in  the  folds  of 
a  letter. 

Disease  thus  caused,  it  must  not  be  forgotten,  is  an  illustra- 
tion of  the  struggle  for  existence  and  the  survival  of  the  fittest. 
If  the  cells  of  an  organism  are  mightier  than  the  bacteria,  the 
latter  are  overwhelmed  ;  but  if  the  bacteria  are  too  great  in 
numbers  or  more  vigorous,  the  cells  must  yield  ;  the  battle  may 
waver — now  dangerous  disease,  now  improvement — but  in  the 
end  the  strongest  in  this,  as  in  other  instances,  prevail. 


GENERAL  BIOLOGY.  21 

UNICELLULAR   ANIMALS   WITH   DIFFERENTIATION 
OF   STRUCTURE. 

The  Bell- Animalcule  (Vorticella). 

Amoeba  is  an  example  of  a  one-celled  animal  with  little  per- 
ceptible differentiation  of  structure  or  corresponding-  division 
of  physiological  labor.  This  is  not,  however,  the  case  with  all 
unicellular  animals,  and  we  proceed  to  study  one  of  these  with 
considerable  development  of  -both.  The  Bell  -  animalcule  is 
found  in  both  fresh  and  salt  water,  either  single  or  in  groups. 
It  is  anchored  to  some  object  by  a  rope-like  stalk  of  clear  pro- 
toplasm, that  has  a  spiral  appearance  when  contracted  ;  and 
which,  with  a  certain  degree  of  regularity,  shortens  and  length 
ens  alternately,  suggesting  that  more  definite  movement  (con- 
traction) of  the  form  of  protoplasm  known  as  muscle,  to  be 
studied  later. 

The  body  of  the  creature  is  bell-shaped,  hence  its  name  ;  the 
bell  being  provided  with  a  thick  everted  lip  (peristome),  covered 
with  bristle-like  extensions  of  the  protoplasm  (cilia),  which  are 
in  almost  constant  rhythmical  motion.  Covering  the  mouth  of 
the  bell  is  a  lid,  attached  by  a  hinge  of  protoplasm  to  the  body, 
which  may  be  raised  or  lowered  A  wide,  funnel-like  depres- 
sion {oesophagus)  leads  into  the  softer  substance  within  which 
it  ends  blindly.  The  outer  part  of  the  animal  (cuticula)  is 
denser  and  more  transparent  than  any  other  part  of  the  whole 
creature  ;  next  to  this  is  a  portion  more  granular  and  of  inter- 
mediate transparency  between  the  external  and  innermost  por- 
tions (cortical  layer).  Below  the  disk  is  a  space  (contractile 
vesicle)  filled  with  a  thin,  clear  fluid,  which  may  be  seen  to  en- 
large slowly,  and  then  to  collapse  suddenly.  When  the  Vorti- 
cella is  feeding,  these  vesicles  may  contain  food-particles,  and 
in  the  former,  apparently,  digestion  goes  on.  Such  food  vacu- 
oles (vesicles)  may  circulate  up  one  side  of  the  body  of  the  ani- 
mal and  down  the  other.  Their  exact  significance  is  not  known, 
but  it  would  appear  as  if  digestion  went  on  within  them  ;  and 
possibly  the  clear  fluid  with  which  they  are  filled  may  be  a  spe- 
cial secretion  with  solvent  action  on  food. 

Situated  somewhat  centrally  is  a  horseshoe-shaped  body,  with 
well-defined  edges,  which  stains  more  readily  than  the  rest  of  the 
cell,  indicating  a  different  chemical  composition ;  and,  from  the 
prominent  part  it  takes  in  the  reproductive  and  other  functions 
of  the  creature,  it  may  be  considered  the  nucleus  (endoplast). 


22 


COMPARATIVE   PHYSIOLOGY. 


Multiplication  of  the  species  is  either  by  gemmation  or  by 
fission.      In   the  first  case  the  nucleus  divides  and  the  frag- 


Fig.  37. 


Fig.  40. 


Fig.  38. 


Fig.  39. 


Figs.  34  to  40.— In  the  figures  d  denotes  disk  ;  p, 
peristome;  vc,  contractile  vacuole;  vf,  food- 
vacuole;  vs,  vestibule;  cf,  contractile  fiber; 
c,  cyst;  nc,  nucleus;  cl,  c'ilium. 

Fig.  34. — A  group  of  vorticellte  showing  the  crea- 
ture in  various  positions  (A,  3). 

Fig.  35. — The  same,  in  the  extended  and  in  the 
retracted  state.    (Surface  views.) 

Fig.  36.— Shows  food-vacuoles;  one  in  the  act  of 
ingestion. 

Fig.  37. — A  vorticella,  in  which  the  process  of 
multiplication  by  fission  is  begun. 

Fig.  38. — The  results  of  fission;  the  production 
of  two  individuals  of  unequal  size. 

Fig.  39.— Illustration  of  reproduction  by  conju- 
gation. 

Fig.  40.— An  encysted  vorticella. 


Fig.  35, 


ments  are  transformed  into  locomotive  germs;  in  the  latter 
the  entire  animal,  including  the  nucleus,  divides  longitudi- 
nally, each  half  becoming  a  similar  complete,  independent  or- 
ganism. Still  another  method  of  reproduction  is  known.  A 
more  or  less  globular  body  encircled  with  a  ring  of  cilia  and 
of  relatively  small  size  may  sometimes  be  seen  attached  to 
the  usual  form  of  Vorticella,  with  which  it  finally  becomes 
blended  into  one  mass.     This  seems  to  foreshadow  the  "  sexual 


GENERAL    BIOLOGY.  23 

conjugation  "  of  higher  forms,  and  is  of  great  biological  sig- 
nificance. 

Vorticella  may  pass  into  an  encysted  and  quiescent  stage  for 
an  indefinite  period  and  again  become  active.  The  history  of 
the  Bell-animalcule  is  substantially  that  of  a  vast  variety  of 
one-celled  organisms  known  as  Infusoria,  to  which  Amoeba 
itself  belongs.  It  will  be  observed  that  the  resemblance  of  this 
organism  to  Amoeba  is  very  great;  it  is,  however,  introduced 
here  to  illustrate  an  advance  in  differentiation  of  structure ;  and 
to  show  how,  with  the  latter,  there  is  usually  a  physiological 
advance  also,  since  there  is  additional  functional  progress  or 
division  of  labor ;  but  still  the  whole  of  the  work  is  done  with- 
in one  cell.  Amoeba  and  Vorticella  are  both  factories  in  which 
all  of  the  work  is  done  in  one  room,  but  in  the  latter  case  the 
machinery  is  more  complex  than  in  the  former ;  there  are  cor- 
respondingly more  processes,  and  each  is  performed  with  greater 
perfection.  Thus,  food  in  the  case  of  the  Bell-animalcule  is 
swept  into  the  gullet  by  the  currents  set  up  by  the  multitudes 
of  vibrating  arms  around  this  opening  and  its  immediate  neigh- 
borhood ;  the  contractile  vesicles  play  a  more  prominent  part ; 
and  the  waste  of  undigested  food  is  ejected  at  a  more  definite 
portion  of  the  body,  the  floor  of  the  oesophagus ;  while  all  the 
movements  of  the  animal  are  rhythmical  to  a  degree  not  exem- 
plified in  such  simple  forms  as  Amoeba;  not  to  mention  its 
various  resources  for  multiplication  and,  therefore,  for  its 
perpetuation  and  permanence  as  a  species.  It,  too,  like  all  the 
unicellular  organisms  we  have  been  considering,  is  susceptible 
of  very  wide  distribution,  being  capable  of  retaining  vitality  in 
the  driedf  state,  so  that  these  infusoria  may  be  carried  in  vari- 
ous directions  by  winds  in  the  form  of  microscopic  dust. 

MULTICELLULAR   ORGANISMS. 

The  Fresh- Water  Polyps  {Hydra  viridis  ;  Hydra  fused). 

The  comparison  of  an  animal  so  simple  in  structure,  though 
made  up  of  many  cells,  as  the  Polyp,  with  the  more  complex 
organizations  with  which  we  shall  have  especially  to  deal,  may 
be  fitly  undertaken  at  this  stage.  The  Polyps  are  easily  obtain- 
able from  ponds  in  which  they  are  found  attached  to  various 
kinds  of  weeds.  To  the  naked  eye,  they  resemble  translucent 
masses  of  jelly  with  a  greenish  or  reddish  tinge.  They  range 
in  size  from  one  quarter  to  one  half  an  inch ;  are  of  an  elongated 


COMPARATIVE  PHYSIOLOGY. 


Fio.  40, 


GENERAL  BIOLOGY.  25 

Figs.  41  to  46.— In  the  figures  ec  denotes  ectoderm;  en,  endoderm.;  /,  tentacle;  hp, 
hypostome;  /,  foot;  te,  testes;  ov,  ovary;  ps,  pseudopodiam;  ec',  larger  ectoderm 

cells:  ne\  larger  nematocysts  before  rupture;  cp,  Kleinenberg's  fibers;  <■./.  sup- 
porting lamella;  c/„  chlorophyl-forruing  bodies;  e,  cilium. 

Fig.  41. — The  green  hydra,  at  the  maximum  of  contraction  and  elongation  of  its  body. 
The  creature  is  represented  in  the  act  of  seizing  a  small  crustacean  (A,  2). 

Fig.  42. — Transverse  section  across  the  body  of  a  hydra,  in  the  digestive  cavity  of 
which  a  small  crustacean  is  represented. 

Fig.  43.— The  leading  types  of  thread-cells,  after  liberation  from  the  body  (P,  3).  The 
cells  are  represented  in  the  active  and  the  resting  conditions;  in  the  former  all  the 
parts  are  more  distinctly  seen  in  consequence  of  the  necessary  eversion. 

Fig.  44.— Small  portion  of  a  transverse  section  across  the  bodv  of  a  green  hydra 
(D,  3). 

Fig.  45. — A  large  brown  hydra  bearing  at  the  same  time  buds  produced  asexually  and 
sexual  organs. 

Fig.  4tj. — Larger  cells  of  the  ectoderm  isolated.  Note  the  processes  of  the  cells  or 
Kleinenberg's  fibers  (F,  3). 

All  the  cuts  on  pages  9  to  34  have  been  selected  from  Howes'  Atlas  of  Biology. 

cylindrical  form ;  provided  at  the  oral  extremity  with  thread- 
like tenacles  of  considerable  length,  which  are  slowly  moved 
about  in  all  directions ;  but  they  and  the  entire  body  may  short- 
en rapidly  into  a  globular  mass.  They  are  usually  attached  at 
the  opposite  (aboral)  pole  to  some  object,  but  may  float  free,  or 
slowly  crawl  from  place  to  place.  It  may  be  observed,  under 
the  microscope,  that  the  tenacles  now  and  then  embrace  some 
living  object,  convey  it  toward  an  opening  (mouth)  near  their 
base,  from  which,  from  time  to  time,  refuse  material  is  cast  out. 
It  may  be  noticed,  too,  that  a  living  object  within  the  touch  of 
these  tenacles  soon  loses  the  power  to  struggle,  which  is  owing 
to  the  peculiar  cells  (nettle-cells,  urticating  capsules,  nemato- 
cysts) with  which  they  are  abundantly  provided,  and  which  se- 
crete a  poisonovis  fluid  that  paralyzes  prey. 

The  mouth  leads  into  a  simple  cavity  (coelom)  in  which 
digestion  proceeds.  The  green  color  in  Hydra  viridis,  and  the 
red  color  of  Hydra  fusca,  is  owing  to  the  presence  of  chlorophyl, 
the  function  of  which  is  not  known.  Hydra  is  structurally  a 
sac,  made  up  of  two  layers  of  cells,  an  outer  (ectoderm)  and 
an  inner  (endoderm):  the  tentacles  being  repetitions  of  the 
scructure  of  the  main  body  of  the  animal,  and  so  hollow  and 
composed  of  two  cell  layers.  Speaking  generally,  the  outer 
layer  is  devoted  to  obtaining  information  of  the  surroundings  ; 
the  inner  to  the  work  of  preparing  nutriment,  and  probably, 
also,  discharging  waste  matters,  in  which  latter  assistance  is 
also  received  from  the  outer  layer.  As  digestion  takes  place 
largely  within  the  cells  themselves,  or  is  intracellular,  we  are 
reminded  of  Vorticella  and  still  more  of  Amoeba.  There  is  in 
Hydra  a  general  advance  in  development,  but  not  very  much 
individual  cell  specialization.  That  of  the  urticating  capsules  is 
one  of  the  best  examples  of  such  specialization  in  this  creature. 


26  COMPARATIVE   PHYSIOLOGY. 

A  Polyp  is  like  a  colony  of  Amoebae  in  which  some  division  of 
labor  (function)  has  taken  place  ;  a  sort  of  biological  state  in 
which  every  individual  is  nearly  equal  to  his  neighbor,  but 
somewhat  more  advanced  than  those  neighbors  not  members  of 
the  organization. 

But  in  one  respect  the  Polyps  show  an  enormous  advance. 
Ordinarily  when  nourishment  is  abundant  Hydra  multiplies  by 
budding,  and  when  cut  into  portions  each  may  become  a  com- 
plete individual.  However,  under  other  circumstances,  near 
the  bases  of  the  tentacles  the  body  wall  may  protrude  into  little 
masses  (tes  es),  in  which  cells  of  peculiar  formation  (sperma- 
tozoa) arise,  and  are  eventually  set  free  and  unite  with  a  cell- 
{ovum)  formed  in  a  similar  protrusion  of  larger  size  (ovary). 
Here,  then,  is  the  first  instance  in  which  distinctly  sexual  repro- 
duction has  been  met  in  our  studies  of  the  lower  forms  of  life. 
This  is  substantially  the  same  process  in  Hydra  as  in  mammals. 
But,  as  both  male  and  female  cells  are  produced  by  the  same 
individual,  the  sexes  are  united  (hermaphroditism)  ;  each  is  at 
once  male  and  female. 

Any  one  watching  the  movements  of  a  Polyp,  and  compar- 
ing it  with  those  of  a  Bell-animalcule,  will  observe  that  the 
former  are  much  less  machine-like ;  have  greater  range  ;  seem 
to  be  the  result  of  a  more  deliberate  choice ;  are  better  adapted 
to  the  environment,  and  calculated  to  achieve  higher  ends.  In 
the  absence  of  a  nervous  system  it  is  not  easy  to  explain  how 
one  part  moves  in  harmony  with  another,  except  by  that  pro- 
cess which  seems  to  be  of  such  wide  application  in  nature,  adap- 
tation from  habitual  simultaneous  effects  on  a  protoplasm  capa- 
ble of  responding  to  stimuli.  When  one  process  of  an  Amoeba 
is  touched,  it  is  likely  to  withdraw  all.  This  we  take  to  be  due 
to  influences  radiating  through  molecular  movement  to  other 
parts ;  the  same  principle  of  action  may  be  extended  to  Hydra. 
The  oftener  any  molecular  movement  is  repeated,  the  more  it 
tends  to  become  organized  into  regularity,  to  become  fixed  in 
its  mode  of  action  ;  and  if  we  are  not  mistaken  this  is  a  funda- 
mental law  throughout  the  entire  world  of  living  things,  if  not 
of  all  things  animate  and  inanimate  alike.  To  this  law  we 
shall  return. 

But  Hydra  is  a  creature  of  but  very  limited  specializations; 
there  are  neither  organs  of  circulation,  respiration,  nor  excretion, 
if  we  exclude  the  doubtful  case  of  the  thread-cells  (urticating 
capsules).     The  animal  breathes  by  the  entire  surface  of  the 


GENERAL  BIOLOGY.  27 

body  ;  nourishment  passes  from  cell  to  cell,  and  waste  is  dis- 
charged into  the  water  surrounding  the  creature  from  all  cells, 
though  probably  not  quite  equally.  All  parts  are  not  digestive, 
respiratory,  etc.,  to  the  same  degree,  and  herein  does  it  differ 
greatly  from  Amoeba  or  even  Vorticella,  though  fuller  knowl- 
edge will  likely  modify  our  views  of  the  latter  two  and  similar 
organisms  in  this  regard. 

THE  CELL  RECONSIDERED. 

Having  now  studied  certain  one-celled  plants  and  animals, 
and  some  very  simple  combinations  of  cells  (molds,  etc.),  it  will 
be  profitable  to  endeavor  to  generalize  the  lessons  these  humble 
organisms  convey  ;  for,  as  will  be  constantly  seen  in  the  study  of 
the  higher  forms  of  life  of  which  this  work  proposes  to  treat 
principally,  the  same  laws  operate  as  in  the  lowliest  living  creat- 
ures. The  most  complex  organism  is  made  up  of  tissues,  which 
are  but  cells  and  then*  products,  as  houses  are  made  of  bricks, 
mortar,  wood,  and  a  few  other  materials,  however  large  or  elab- 
orate. 

The  student  of  physiology  who  proceeds  scientifically  must 
endeavor,  in  investigating  the  functions  of  each  organ,  to  learn 
the  exact  behavior  of  each  cell  as  determined  by  its  own  inherent 
tendencies,  and  modified  by  the  action  of  neighboring  cells. 
The  reason  why  the  function  of  one  organ  differs  from  that  of 
another  is  that  its  cells  have  departed  in  a  special  direction  from 
those  properties  common  to  all  cells,  or  have  become  function- 
ally differentiated.  But  such  a  statement  has  no  meaning  un- 
less it  be  well  understood  that  cells  have  certain  properties  in 
common.  This  is  one  of  the  lessons  imparted  by  the  preceding 
studies  which  we  now  review.  Briefly  stated  in  language  now 
extensively  used  in  works  on  biology,  the  common  properties  of 
cells' (protoplasm),  whether  animal  or  vegetable,  whether  consti- 
tuting in  themselves  entire  animals  or  plants,  or  forming  the 
elements  of  tissues,  are  these  :  The  collective  chemical  processes 
associated  with  the  vital  activities  of  cells  are  termed  its  metab- 
olism. Metabolism  is  constructive  when  more  complex  com- 
pounds are  formed  from  simple  ones,  as  when  the  Protococcus- 
cell  builds  up  its  protoplasm  out  of  the  simple  materials,  found  in 
rain-water,  which  makes  up  its  food.  Metabolism,  is  destructive 
when  the  reverse  process  takes  place.  The  results  of  this  process 
are  eliminated  as   excreta,  or  useless  and  harmful  products. 


28  COMPARATIVE   PHYSIOLOGY. 

Since  all  the  vital  activities  of  cells  can  only  be  manifested  when 
supplied  with  food,  it  follows  that  living  organisms  convert  po- 
tential or  possible  energy  into  kinetic  or  actual  energy.  When 
lifeless,  immobile  matter  is  taken  in  as  food  and,  as  a  result,  is 
converted  by  a  process  of  assimilation  into  the  protoplasm  of  the 
cell  using  it,  we  have  an  example  of  potential  being  converted 
into  actual  energy,  for  one  of  the  properties  of  all  protoplasm  is 
its  contractility.  Assimilation  implies,  of  course,  the  absorp- 
tion of  what  is  to  be  used,  with  rejection  of  waste  matters. 

The  movements  of  protoplasm  of  whatever  kind,  when  due 
to  a  stimulus,  are  said  to  indicate  irritability ;  while,  if  inde- 
pendent of  any  external  source  of  excitation,  they  are  denomi- 
nated automatic. 

Among  agents  that  modify  the  action  of  all  kinds  of  proto- 
plasm are  heat,  moisture,  electricity,  light,  and  others  in  great 
variety,  both  chemical  and  mechanical.  It  can  not  be  too  well 
remembered  that  living  things  are  what  they  are,  neither  by 
virtue  of  their  own  organization  alone  nor  through  the  action 
of  their  environment  alone  (else  would  they  be  in  no  sense  dif- 
ferent from  inanimate  things),  but  because  of  the  relation  of 
the  organization  to  the  surroundings. 

Protoplasm,  then,  is  contractile,  irritable,  automatic,  absorp- 
tive, secretory  (and  excretory),  metabolic,  and  reproductive. 

But  when  it  is  affirmed  that  these  are  the  fundamental  prop- 
erties of  all  protoplasm,  the  idea  is  not  to  be  conveyed  that  cells 
exhibiting  these  properties  are  identical  biologically.  No  two 
masses  of  protoplasm  can  be  quite  alike,  else  would  there  be  no 
distinction  in  physiological  demeanor — no  individuality.  Every 
cell,  could  we  but  behold  its  inner  molecular  mechanism,  differs 
from  its  neighbor.  When  this  difference  reaches  a  certain  de- 
gree in  one  direction,  we  have  a  manifest  differentiation  leading 
to  physiological  division  of  labor,  which  may  now  with  advan- 
tage be  treated  in  the  following  section. 

THE   ANIMAL   BODY. 

An  animal,  as  we  have  learned,  may  be  made  up  of  a  single 
cell  in  which  each  part  performs  much  the  same  work ;  or,  if 
there  be  differences  in  function,  they  are  ill-defined  as  compared 
with  those  of  higher  animals.  The  condition  of  tilings  in  such 
an  animal  as  Amoeba  may  be  compared  to  a  civilized  commu- 
nity in  a  very  crude  social  condition.     When  each  individual 


GENERAL   BIOLOGY.  29 

tries  to  perform  every  office  for  himself,  he  is  at  once  carpenter, 
blacksmith,  shoemaker,  and  much  more,  with  the  natural  re- 
sult that  he  is  not  efficient  in  any  one  direction.  A  community 
may  be  judged  in  regard  to  its  degree  of  advancement  by  the 
amount  of  division  of  labor  existing  within  it.  Thus  is  it  with 
the  animal  body.  We  find  in  such  a  creature  as  the  fresh-water 
Hydra,  consisting  of  two  layers  of  cells  forming  a  simple  sac,  a 
slight  amount  of  advancement  on  Amoeba.  Its  external  surface 
no  longer  serves  for  inclosure  of  food,  but  it  has  the  simplest 
form  of  mouth  and  tentacles.  Each  of  the  cells  of  the  internal 
layer  seems  to  act  as  a  somewhat  improved  or  specialized  Amoe- 
ba, while  in  those  of  the  outer  layer  we  mark  a  beginning  of 
those  functions  which  taken  collectively  give  the  higher  ani- 
mals information  of  the  surrounding  world. 

Looking  to  the  existing  state  of  things  in  the  universe,  it  is 
plain  that  an  animal  to  attain  to  high  ends  must  have  powers 
of  rapid  locomotion,  capacity  to  perceive  what  makes  for  its  in- 
terest, and  ability  to  utilize  means  to  obtain  this  when  perceived. 
These  considerations  demand  that  an  animal  high  in  the  scale 
of  being  should  be  provided  with  limbs  sufficiently  rigid  to  sup- 
port its  weight,  moved  by  strong  muscles,  which  must  act  in 
harmony.  But  this  implies  abundance  of  nutriment  duly  pre- 
pared and  regularly  conveyed  to  the  bones  and  muscles.  All 
this  would  be  useless  unless  there  was  a  controlling  and  ener- 
gizing system  capable  both  of  being  impressed  and  originating 
impressions.  Such  is  found  in  the  nerves  and  nerve- centers. 
Again,  in  order  that  this  mechanism  be  kept  in  good  running 
order,  the  waste  of  its  own  metabolism,  which  chokes  and  poi- 
sons, must  be  got  rid  of— hence  the  need  of  excretory  apparatus. 
In  order  that  the  nervous  system  may  get  sufficient  informa- 
tion of  the  world  around,  the  surface  of  the  body  must  be  pro- 
vided with  special  message-receiving  offices  in  the  form  of 
modified  nerve-endings.  In  short,  it  is  seen  that  an  animal  as 
high  in  the  scale  as  a  mammal  must  have  muscular,  osseous 
(aud  connective),  digestive,  circulatory,  excretory,  and  nervous 
tissues ;  and  to  these  may  be  added  certain  forms  of  protective 
tissues,  as  hair,  nails,  etc.  * 

Assuming  that  the  student  has  at  least  some  general  knowl- 
edge of  the  structure  of  these  various  tissues,  we  propose  to  tell 
in  a  simple  way  the  whole  physiological  story  in  brief. 

The  blood  is  the  source  of  all  the  nourishment  of  the  organ- 
ism, including  its  oxygen  supply,  and  is  carried  to  eveiy  part  of 


30  COMPARATIVE   PHYSIOLOGY. 

the  body  through  elastic  tubes  which,  continually  branching 
and  becoming  gradually  smaller,  terminate  in  vessels  of  hair- 
like fineness  in  which  the  current  is  very  slow — a  condition  per- 
mitting that  interchange  between  the  cells  surrounding  them 
and  the  blood  which  may  be  compared  to  a  process  of  barter, 
the  cells  taking  nutriment  and  oxygen,  and  giving  (excreting) 
in  return  carbonic  anhydride.  From  these  minute  vessels  the 
blood  is  conveyed  back  toward  the  source  whence  it  came  by 
similar  elastic  tubes  which  gradually  increase  in  size  and  be- 
come fewer.  The  force  which  directly  propels  the  blood  in  its 
onward  course  is  a  muscular  pump,  with  both  a  forcing  and 
suction  action,  though  chiefly  the  former.  The  flow  of  blood 
is  maintained  constant  owing  to  the  resistance  in  the  smaller 
tubes  on  the  one  hand  and  the  elastic  recoil  of  the  larger  tubes 
on  the  other  ;  while  in  the  returning  vessels  the  column  of 
blood  is  suppoi'ted  by  elastic  double  gates  which  so  close  as  to 
prevent  reflux.  The  oxygen  of  the  blood  is  carried  in  disks  of 
microscopic  size  which  give  it  up  in  proportion  to  the  needs  of 
the  tissues  past  which  they  are  cai^ried. 

But  in  reality  the  tissues  of  the  body  are  not  nourished 
directly  by  the  blood,  but  by  a  fluid  derived  from  it  and  resem- 
bling it  greatly  in  most  particulars.  This  fluid  bathes  the  tis- 
sue-cells on  all  sides.  It  also  is  taken  up  by  tubes  that  convey 
it  into  the  blood  after  it  has  passed  through  little  factories 
(lymphatic  glands),  in  which  it  undergoes  a  regeneration. 
Since  the  tissues  are  impoverishing  the  blood  by  withdrawal  of 
its  constituents,  and  adding  to  it  what  is  no  longer  useful,  and 
is  in  reality  poisonous,  it  becomes  necessary  that  new  material 
be  added  to  it  and  the  injurious  components  withdrawn.  The 
former  is  accomplished  by  the  absorption  of  the  products  of 
food  digestion,  and  the  addition  of  a  fresh  supply  of  oxygen 
derived  from  without,  while  the  poisonous  ingredients  that 
have  found  their  way  into  the  blood  are  got  rid  of  through 
processes  that  may  be,  in  general,  compared  to  those  of  a  sew- 
age system  of  a  very  elaborate  character.  To  explain  this  re- 
generation of  the  blood  in  somewhat  more  detail,  we  must  first 
consider  the  fate  of  food  from  the  time  it  enters  the  mouth  till 
it  leaves  the  tract  of  the  body  in  which  its  preparation  is  car- 
ried on. 

The  food  is  in  the  mouth  submitted  to  the  action  of  a  series 
of  cutting  and  grinding  organs  worked  by  powerful  muscles  ; 
mixed  with  a  fluid  which  changes  the  starchy  part  of  it  into 


GENERAL  BIOLOGY.  31 

sugar,  and  prepares  the  whole  to  pass  further  on  its  course  : 
when  this  has  heen  accomplished,  the  food  is  grasped  and 
squeezed  and  pushed  along  the  tube,  owing  to  the  action  of  its 
own  muscular  cells,  into  a  sac  (stomach),  in  which  it  is  rolled 
about  and  mixed  with  certain  fluids  of  peculiar  chemical  com- 
position derived  from  cells  on  its  inner  surface,  which  trans- 
form the  proteid  part  of  the  food  into  a  form  susceptible  of 
ready  use  (absorption).  When  this  saccular  organ  has  done 
its  share  of  the  work,  the  food  is  moved  on  by  the  action  of 
the  muscles  of  its  walls  into  a  very  long  portion  of  the  tract 
in  which,  in  addition  to  processes  carried  on  in  the  mouth  and 
stomach,  there  are  others  which  transform  the  food  into  a  con- 
dition in  which  it  can  pass  into  the  blood.  Thus,  all  of  the 
food  that  is  susceptible  of  changes  of  the  kind  described  is  acted 
upon  somewhere  in  the  long  tract  devoted  to  this  task.  But 
there  is  usually  a  remnant  of  indigestible  material  which  is 
finally  evacuated.  How  is  the  prepared  material  conveyed  into 
the  blood  ?  In  part,  directly  through  the  walls  of  the  minutest 
blood-vessels  distributed  throughout  the  length  of  this  tube  ; 
and  in  part  through  special  vessels  with  appropriate  cells  cov- 
ering them  which  act  as  minute  porters  (villi). 

The  impure  blood  is  carried  periodically  to  an  extensive  sur- 
face, usually  much  folded,  and  there  exposed  in  the  hair-like 
tubes  referred  to  before,  and  thus  parts  with  its  excess  of  car- 
bon dioxide  and  takes  up  fresh  oxygen.  But  all  the  functions 
described  do  not  go  on  in  a  fixed  and  invariable  manner,  but 
are  modified  somewhat  according  to  circumstances.  The  for- 
cing-pump of  the  circulatory  system  does  not  always  beat 
equally  fast  ;  the  smaller  blood-vessels  are  not  always  of  the 
same  size,  but  admit  more  or  less  blood  to  an  organ  according 
to  its  needs. 

This  is  all  accomplished  in  obedience  to  the  commands  car- 
ried from  the  brain  and  spinal  cord  along  the  nerves.  All 
movements  of  the  limbs  and  other  parts  are  executed  in  obe- 
dience to  its  behests ;  and  in  order  that  these  may  be  in  accord- 
ance with  the  best  interests  of  each  particular  organ  and  the 
whole  animal,  the  nervous  centers.  Which  may  be  compared  to 
the  chief  officers  of,  say,  a  telegraph  or  railway  system,  are  in 
constant  receipt  of  information  by  messages  carried  onward 
along  the  nerves.  The  command  issuing  is  always  related  to 
the  information  arriving. 

All  those  parts  commonly  known  as  sense-organs — the  eye, 


32  COMPARATIVE   PHYSIOLOGY. 

ear,  nose,  tongue,  and  the  entire  surface  of  the  body — are  faith- 
ful reporters  of  facts.  They  put  the  inner  and  outer  worlds  in 
communication,  and  without  them  all  higher  life  at  least  must 
cease,  for  the  organism,  like  a  train  directed  by  a  conductor  that 
disregards  the  danger-signals,  must  work  its  own  destruction. 
Without  going  into  further  details,  suffice  it  to  say  that  the  pro- 
cesses of  the  various  cells  are  subordinated  to  the  general  good 
through  the  nervous  system,  and  that  susceptibility  of  proto- 
plasm to  stimuli  of  a  delicate  kind  which  enables  each  cell  to 
adapt  to  its  surroundings,  including  the  influence  of  remote  as 
well  as  neighboring  cells.  Without  this  there  could  be  no 
marked  advance  in  organisms,  no  differentiation  of  a  pro- 
nounced character,  and  so  none  of  that  physiological  division 
of  labor  which  will  be  inferred  from  our  brief  description  of 
the  functions  of  a  mammal.  The  whole  of  physiology  but 
illustrates  this  division  of  labor. 

It  is  hoped  that  the  above  account  of  the  working  of  the  ani- 
mal body,  brief  as  it  is,  may  serve  to  show  the  connection  of 
one  part  functionally  with  another,  for  it  is  much  more  impor- 
tant that  this  should  be  kept  in  mind  throughout,  than  that  all 
the  details  of  any  one  function  should  be  known. 

LIVING  AND   LIFELESS   MATTER. 

In  order  to  enable  the  student  the  better  to  realize  the  na- 
ture of  living  matter  or  protoplasm,  and  to  render  clearer  the 
distinction  between  the  forms  that  belong  to  the  organic  and 
inorganic  worlds  respectively,  we  shall  make  some  comparisons 
in  detail  which  it  is  hoped  may  accomplish  this  object. 

A  modern  watch  that  keeps  correct  time  must  be  regarded 
as  a  wonderful  object,  a  marvelous  triumph  of  human  skill. 
That  it  has  aroused  the  awe  of  savages,  and  been  mistaken  for  a 
living  being,  is  not  surprising.  But,  admirable  as  is  the  result 
attained  by  the  mechanism  of  a  watch,  it  is,  after  all,  composed 
of  but  a  few  metals,  etc.,  chiefly  in  fact  of  two,  brass  and  steel  ; 
these  are,  however,  made  up  into  a  great  number  of  different 
parts,  so  adapted  to  one  another  as  to  work  in  unison  and  ac- 
complish the  desired  object  of  indicating  the  time  of  day. 

Now,  however  well  constructed  the  watch  may  be,  there  are 
waste,  wear  and  tear,  which  will  manifest  themselves  more  and 
more,  until  finally  the  machine  becomes  worthless  for  the  pur- 
pose of  its  construction.    If  this  mechanism  possessed  the  power 


GENERAL  BIOLOGY.  33 

of  adapting-  from  without  foreign  matter  so  as  to  construct  it 
it  into  steel  and  brass,  and  arrange  this  just  when  required,  it 
would  imitate  a  living  organism  ;  but  this  it  can  not  do,  nor  is 
its  waste  chemically  different  from  its  component  metals  ;  it 
does  not  break  up  brass  and  steel  into  something  wholly  differ- 
ent. In  one  particular  it  does  closely  resemble  living  things, 
in  that  it  gradually  deteriorates  ;  but  the  degradation  of  a  liv- 
ing cell  is  tbe  consequence  of  an  actual  change  in  its  compo- 
nent parts,  commonly  a  fatty  degeneration.  The  one  is  a  real 
transformation,  the  other  mere  wear. 

Had  the  watch  the  power  to  give  rise  to  a  new  one  like  itself 
by  any  process,  especially  a  process  of  division  of  itself  into  two 
parts,  we  should  have  a  parallel  with  living  forms  ;  but  the 
watch  can  not  even  renew  its  own  parts,  much  less  give  rise  to 
a  second  mechanism  like  itself.  Here,  then,  is  a  manifest  dis- 
tinction between  living  and  inanimate  things. 

Suppose,  further,  that  the  watch  was  so  constructed  that, 
after  the  lapse  of  a  certain  time,  it  underwent  a  change  in  its 
inner  machinery  and  perhaps  its  outer  form,  so  as  to  be  scarcely 
recognizable  as  the  same  ;  and  that  as  a  result,  instead  of  indi- 
cating the  hours  and  minutes  of  a  time-reckoning  adapted  to 
the  inhabitants  of  our  globe,  it  indicated  time  in  a  wholly  dif- 
ferent way  ;  that  after  a  series  of  such  transformations  it  fell  to 
pieces— took  the  original  form  of  the  metals  from  which  it  was 
constructed — we  should  then  have  in  this  succession  of  events  a 
parallel  with  the  development,  decline,  and  death  of  living  or- 
ganisms. 

In  another  particular  our  illustration  of  a  watch  may  serve 
a  useful  purpose.  Suppose  a  watch  to  exist,  the  works  of  which 
are  so  concealed  as  to  be  quite  inaccessible  to  our  vision,  so  that 
all  we  know  of  it  is  that  it  has  a  mechanism  which  when  in 
action  we  can  hear,  and  the  result  of  which  we  perceive  in  the 
movements  of  the  hands  on  the  face  ;  we  should  then  be  in  the 
exact  position  in  reference  to  the  watch  that  we  now  are  as  re- 
gards the  molecular  movements  of  protoplasm.  On  the  latter 
the  entire  behavior  of  living  matter  depends  ;  yet  it  is  abso- 
lutely hidden  from  us. 

We  know,  too,  that  variations  must  be  produced  in  the 
mechanism  of  time-pieces  by  temperature,  moisture,  and  other 
influences  of  the  environment,  resulting  in  altered  action.  The 
same,  as  will  be  shown  in  later  chapters,  occurs  in  protoplasm. 
Tins,  too,  is  primarily  a  molecular  effect.  If  the  works  of 
3 


34  COMPARATIVE  PHYSIOLOGY. 

watches  were  beyond  observation,  we  should  not  be  able  to  state 
exactly  how  the  variations  observed  in  different  kinds,  or  even 
different  individuals  of  the  same  kind  occurred,  though  these 
differences  might  be  of  the  most  marked  character,  such  as  any 
one  could  recognize.  Here  once  more  we  refer  the  differ- 
ences to  the  mechanism.  So  is  it  with  living  beings  :  the  ulti- 
mate molecular  mechanism  is  unknown  to  us. 

Could  we  but  render  these  molecular  movements  visible  to 
our  eyes,  we  should  have  a  revelation  of  far  greater  scientific 
importance  than  that  unfolded  by  the  recent  researches  into 
those  living  forms  of  extreme  minuteness  that  swarm  every- 
where as  dust  in  a  sunbeam,  and,  as  will  be  learned  later,  are 
often  the  source  of  deadly  disease.  Like  the  movements  of  the 
watch,  the  activities  of  protoplasm  are  ceaseless.  A  watch  that 
will  not  run  is,  as  such,  worthless — it  is  mere  metal — has  under- 
gone an  immense  degradation  in  the  scale  of  values  ;  so  proto- 
plasm is  no  longer  protoplasm  when  its  peculiar  molecular 
movements  cease  ;  it  is  at  once  degraded  to  the  rank  of  dead 
matter. 

The  student  may  observe  that  each  of  the  four  propositions, 
embodying  the  fundamental  properties  of  living  matter,  stated 
in  the  preceding  chapter,  have  been  illustrated  by  the  simile  of 
a  watch.  Such  an  illustration  is  necessarily  crude,  but  it  helps 
one  to  realize  the  meaning  of  truths  which  gather  force  with 
each  living  form  studied  if  regarded  aright  ;  and  it  is  upon  the 
realization  of  truth  that  mental  growth  as  well  as  practical 
efficiency  depends. 

CLASSIFICATION   OF   THE  ANIMAL  KINGDOM. 

There  are  human  beings  so  low  in  the  scale  as  not  to  possess 
such  general  terms  as  tree,  while  they  do  employ  names  for  dif- 
ferent kinds  of  trees.  The  use  of  such  a  word  as  "  tree  "  im- 
plies generalization,  or  the  abstraction  of  a  set  of  qualities  from 
the  things  in  which  they  reside,  and  making  them  the  basis  for 
the  grouping  of  a  multitude  of  objects  by  which  we  are  sur- 
rounded. Manifestly  without  such  a  process  knowledge  must 
be  very  limited,  and  the  world  without  significance  ;  while  in 
proportion  as  generalization  may  be  safely  widened,  is  our 
progress  in  the  unification  of  knowledge  toward  which  science 
is  tending.  But  it  also  follows  that  without  complete  knowl- 
edge there  can  be  no  perfect  classification  of  objects  ;   hence, 


GENERAL  BIOLOGY.  35 

any  classification  must  be  regarded  but  as  the  temporary  creed 
of  science,  to  be  modified  with  the  extension  of  knowledge.  As 
a  matter  of  fact  this  has  been  the  history  of  all  zoological  and 
other  systems  of  arrangement.  The  only  purpose  of  grouping 
is  to  simplify  and  extend  knowledge  ;  this  being  the  case,  it  fol- 
lows that  a  method  of  grouping  that  accomplishes  this  has 
value,  though  the  system  may  be  artificial  that  is  based  on 
resemblances  which,  though  real  and  constant,  are  associated 
with  differences  so  numerous  and  radical  that  the  total  amount 
of  likeness  between  objects  thus  grouped  is  often  less  than  the 
difference.  Such  a  system  was  that  of  Linnaeus,  who  classified 
plants  according  to  the  number  of  stamens,  etc.,  they  bore. 

Seeing  that  animals  which  resemble  each  other  are  of  com- 
mon descent  from  some  earlier  form,  to  establish  the  line  of  de- 
scent is  to  determine  in  great  part  the  classification.  Much  as- 
sistance in  this  direction  is  derived  from  embryology,  or  the 
history  of  the  development  of  the  individual  {ontogeny)  •  so 
that  it  may  be  said  that  the  ontogeny  indicates,  though  it  does 
not  actually  determine,  the  line  of  descent  (phytogeny)  ;  and 
it  is  owing  to  the  importance  of  this  truth  that  naturalists  have 
in  recent  years  given  so  much  attention  to  comparative  embry- 
ology. 

It  will  be  inferred  that  a  natural  system  of  classification  must 
be  based  both  on  function  and  structure,  though  chiefly  on  the 
latter,  since  organs  of  very  different  origin  may  have  a  similar 
function ;  or,  to  express  this  otherwise,  homologous  structures 
may  not  be  analogous  ;  and  homology  gives  the  better  basis  for 
classification.  To  illustrate,  the  wing  of  a  bat  and  a  bird  are 
both  homologous  and  analogous  ;  the  wing  of  a  butterfly  is 
analogous  but  not  homologous  with  these  ;  manifestly,  to  clas- 
sify bats  and  birds  together  would  be  better  than  to  put  birds 
and  insects  in  the  same  group,  thus  leaving  other  points  of  re- 
lationship out  of  consideration. 

The  broadest  possible  division  of  the  animal  kingdom  is  into 
groups,  including  respectively  one-celled  and  many-celled  forms 
— i.  e.,  into  Protozoa  and  Metazoa.  As  the  wider  the  grouping 
the  less  are  differences  considered,  it  follows  that  the  more  sub- 
divided the  groups  the  more  complete  is  the  information  con- 
veyed ;  thus,  to  say  that  a  dog  is  a  metazoan  is  to  convey  a  cer- 
tain amount  of  information  ;  that  he  is  a  vertebrate,  more  ;  that 
he  is  a  mammal,  a  good  deal  more,  because  each  of  the  latter 
terms  includes  the  former. 


36 


COMPARATIVE  PHYSIOLOGY. 


Inverte- 
brata. 


Animal 
Kingdom.   | 


r  Protozoa  (amoeba,  vorticella,  etc.). 

Ccelenterata  (sponges,  jelly-fish,  polyps,  etc.). 
I   Echinodermata  (star-fish,  sea-urchins,  etc.). 
J   Vermes  (worms). 
1   Ai'thropods  (crabs,  insects,  spiders,  etc.). 

Mollusca  (oysters,  snails,  etc.). 

Molluscoidea  (moss-like  animals). 
*-  Tunicata  (ascidians). 


(  Pisces  (fishes). 
Amphibia  (frogs,  menobranehus,  etc.). 
Vertebrata.  \   Keptilia  (snakes,  turtles,  etc.). 
Aves  (birds'). 
Mammalia  (domestic  quadrupeds,  etc.). 


I 


The  above  classification  (of  Claus)  is,  like  all  such  arrange- 
ments, but  the  expression  of  one  out  of  many  methods  of  view- 
ing the  animal  kingdom. 

For  the  details  of  classification  and  for  the  grounds  of  that 
we  have  presented,  we  refer  the  student  to  works  on  zoology  ; 
but  we  advise  those  who  are  not  familiar  with  this  subject, 
when  a  technical  term  is  used,  to  think  of  that  animal  belong- 
ing to  the  group  in  question  with  the  structure  of  which  they 
are  best  acquainted. 

Man's  Place  in  the  Animal  Kingdom. 

It  is  no  longer  the  custom  with  zoologists  to  place  man  in  an 
entirely  separate  group  by  himself  ;  but  he  is  classed  with  the 
primates,  among  which  are  also  grouped  the  anthropoid  apes 
(gorilla,  chimpanzee,  orang,  and  the  gibbon),  the  monkeys  of 
the  Old  and  of  the  New  World,  and  the  lemurs.  So  great  is 
the  structural  resemblance  of  man  and  the  other  primates  that 
competent  authorities  declare  that  there  is  more  difference  be- 
tween the  structure  of  the  most  widely  separated  members  of 
the  group  than  between  certain  of  the  anthropid  apes  and  man. 

The  points  of  greatest  resemblance  between  man  and  the 
anthropoid  apes  are  the  following  :  The  same  number  of  verte- 
brae ;  the  same  general  shape  of  the  pelvis  ;  a  brain  distinguish- 
ing them  from  other  mammals  ;  and  posture,  being  bipeds. 

The  distinctive  characters  are  size,  rather  than  form  of  the 
brain,  that  of  man  being  more  than  twice  as  large  ;  a  relatively 
larger  cranial  base,  by  which,  together  with  the  greater  size  of 
the  jaws,  the  face  becomes  prominent  ;  the  earlier  closure  of 
the  sutures  of  the  cranium,  arresting  the  growth  of  the  brain  ; 
more  developed  canine  teeth  and  difference  in  the  order  of  erup- 
tion of  the  permanent  teeth  ;  the  more  posterior  position  of  the 
foramen  magnum  ;  the  relative  length  of  the  limbs  to  each 


GENERAL  BIOLOGY.  37 

other  and  the  rest  of  the  body  ;  minor  differences  in  the  hands 
and  feet,  especially  the  greater  freedom  and  power  of  apposition 
of  the  great-toe. 

But  the  greatest  distinction  between  man  and  even  his  closest 
allies  among  the  apes  is  to  be  found  in  the  development  to  an 
incomparably  higher  degree  of  his  intellectual  and  moral  na- 
ture, corresponding  to  the  differences  in  weight  and  structure 
of  the  human  brain,  and  associated  with  the  use  of  spoken  and 
written  language  ;  so  that  the  experience  of  previous  genera- 
tions is  not  only  registered  in  the  organism  (heredity),  but  in 
the  readily  available  form  of  books,  etc. 

The  greatest  structural  difference  between  the  races  of  men 
are  referable  to  the  cranium  ;  but,  since  they  all  interbreed 
freely,  they  are  to  be  considered  varieties  of  one  species. 

THE  LAW  OF  PERIODICITY  OR  RHYTHM  IN  NATURE. 

The  term  rhythm  to  most  minds  suggests  music,  poetry,  or 
dancing,  in  all  of  which  it  forms  an  essential  part  so  simple, 
pronounced,  and  uncomplicated  as  to  be  recognized  by  all  with 
ease. 

The  regular  division  of  music  into  bars,  the  recurrence  of 
chords  of  the  same  notes  at  certain  intervals,  of  forte  and  piano, 
seem  to  be  demanded  by  the  very  nature  of  the  human  mind. 
The  same  applies  to  poetry.  Even  a  child  that  can  not  under- 
stand the  language  used,  or  an  adult  listening  to  recitations  in 
an  unknown  tongue,  enjoys  the  flow  and  recurrences  of  the 
sounds.  Dancing  has  in  all  ages  met  a  want  in  human  organi- 
zations, which  is  partly  supplied  in  quieter  moods  by  the  regu- 
larity of  the  steps  in  walking  and  similar  simple  movements. 

But  as  rhythm  runs  through  all  the  movements  of  animals, 
so  is  it  also  found  in  all  literature  and  all  art.  Infinite  variety 
wearies  the  mind,  hence  the  fatigue  felt  by  the  sight-seer.  Re- 
currence permits  of  repose,  and  gratifies  an  established  taste  or 
appetite.  The  mind  delights  in  what  it  has  once  enjoyed,  in 
repetition  within  limits.  Repetition  with  variety  is  manifestly 
a  condition  of  the  growth  and  development  of  the  mind.  This 
seems  to  apply  equally  to  the  body,  for  every  single  function  of 
each  organism,  however  simple  or  complex  it  may  be,  exempli- 
fies this  law  of  periodicity.  The  heart's  action  is  rhythmical 
(beats)  ;  the  blood  flows  in  intermitting  gushes  from  the  central 
pump  ;  the  to-and-fro  movements  of  respiration  are  so  regular 


38  COMPARATIVE  PHYSIOLOGY. 

that  their  cessation  would  arouse  the  attention  of  the  least  in- 
structed ;  food  is  demanded  at  regular  intervals  ;  the  juices  of 
the  digestive  tract  are  poured  out,  not  constantly  but  period- 
ically ;  the  movements  by  which  the  food  is  urged  along  its 
path  are  markedly  rhythmic  ;  the  chemical  processes  of  the 
body  wax  and  wane  like  the  fires  in  a  furnace,  giving  rise  to 
regular  augmentations  of  the  temperature  of  the  body  at  fixed 
hours  of  the  day,  with  corresponding  periods  of  greatest  bodily 
activity  and  the  reverse. 

This  principle  finds  perfect  illustration  in  the  nervous  sys- 
tem. The  respiratory  act  of  the  higher  animals  is  effected 
through  muscular  movements  dependent  on  regular  waves  of 
excitation  reaching  them  along  the  nerves  from  the  central  cells 
which  regularly  discharge  their  forces  along  these  channels. 
Were  not  the  movements  of  the  body  periodic  or  rhythmical, 
instead  of  that  harmony  which  now  prevails,  every  muscular 
act  would  be  a  convulsion,  though  even  in  the  movements  of 
the  latter  there  is  a  highly  compounded  rhythm,  as  a  noise  is 
made  up  of  a  variety  of  musical  notes.  The  senses  are  subject 
to  the  same  law.  The  eye  ceases  to  see  and  the  ear  to  hear  and 
the  hand  to  feel  if  continuously  stimulated;  and  doubtless  in 
all  art  this  law  is  unconsciously  recognized.  That  ceases  to  be 
art  which  fails  to  provide  for  the  alternate  repose  and  excita- 
tion of  the  senses.  The  eye  will  not  tolerate  continuously  one 
color,  the  ear  the  same  sound.  Why  is  a  breeze  on  a  warm  day 
so  refreshing  ?     The  answer  is  obvious. 

Looking  to  the  world  of  animate  nature  as  a  whole,  it  is 
noticed  that  plants  have  their  period  of  sprouting,  flowering, 
seeding,  and  decline;  animals  are  born,  pass  through  various 
stages  to  maturity,  diminish  in  vigor,  and  die.  These  events 
make  epochs  in  the  life-history  of  each  species ;  the  recurrence 
of  which  is  so  constant  that  the  agricultural  and  other  arrange- 
ments even  of  savages  are  planned  accordingly.  That  the  in- 
dividuals of  each  animal  group  have  a  definite  period  of  dura- 
tion is  another  manifestation  of  the  same  law. 

Superficial  observation  suffices  to  furnish  facts  which  show 
that  the  same  law  of  periodicity  is  being  constantly  exemplified 
in  the  world  of  inanimate  things.  The  regular  ebb  and  flow  of 
the  tides ;  the  rise  and  subsidence  of  rivers ;  the  storm  and  the 
calm;  summer  and  winter;  day  and  night — are  all  recurrent, 
none  constant. 

Events  apparently  without  any  regularity,  utterly  beyond 


GENERAL   BIOLOGY.  39 

any  law  of  recurrence,  when  sufficiently  studied  are  found  to 
fall  under  the  same  principle.  Thus  it  took  some  time  to  learn 
that  volcanic  eruptions  occurred  with  a  very  fair  degree  of 
regularity. 

In  judging  of  this  and  all  other  rhythmical  events  it  must 
be  borne  in  mind  that  the  time  standard  is  for  an  irregularity 
that  seems  large,  as  in  the  instance  just  referred  to,  becomes 
small  when  considered  in  relation  to  the  millions  of  years  of 
geological  time;  while  in  the  case  of  music  a  trifling  irregu- 
larity, judged  by  fractions  of  a  second,  can  not  be  tolerated  by 
the  musical  organization — which  is  equivalent  to  saying  that 
the  interval  of  departure  from  exact  regularity  seems  large. 

As  most  of  the  rhythms  of  the  universe  are  compounded  of 
several,  it  follows  that  they  may  seem,  until  closely  studied, 
very  far  from  regular  recurrences.  This  may  be  observed  in 
the  interference  in  the  regularity  of  the  tides  themselves,  the 
daily  changes  of  which  are  subject  to  an  increase  and  decrease 
twice  in  each  month,  owing  to  the  influence  of  the  sun  and 
moon  being  then  either  coincident  or  antagonistic. 

In  the  functions  of  plants  and  animals,  rhythms  must  be- 
come very  greatly  compounded,  doubtless  often  beyond  recog- 
nition. 

Among  the  best  examples  of  rhythm  in  animals  are  daily 
sleep  and  winter  sleep,  or  hibernation ;  yet,  amid  sleep,  dreams 
or  recurrences  of  cerebral  activity  are  common — that  is,  one 
rhythm  (of  activity)  overlies  another  (of  repose).  In  like  man- 
ner many  hibernating  animals  do  not  remain  constantly  in  their 
dormant  condition  throughout  the  winter  months,  but  have 
periods  of  wakefulness ;  the  active  life  recurs  amid  the  life  of 
functional  repose. 

To  return  to  the  world  of  inanimate  matter,  we  find  that  the 
crust  of  the  earth  itself  is  made  of  layers  or  strata  the  result  of 
periods  of  elevation  and  depression,  of  denudation  and  deposi- 
tion, in  recurring  order. 

The  same  law  is  illustrated  by  the  facts  of  the  economic  and 
other  conditions  of  the  social  state  of  civilized  men.  Periods 
of  depression  alternate  with  periods  of  revival  in  commercial 
life. 

There  are  periods  when  many  more  marriages  occur  and 
many  more  children  are  born,  corresponding  with  changes  in 
the  material  conditions  which  influence  men  as  well  as  other 
animals. 


40  COMPARATIVE  PHYSIOLOGY. 

Finally,  and  of  special  interest  to  the  medical  student,  are 
the  laws  of  rhythm  in  disease.  Certain  fevers  have  their  regu- 
lar periods  of  attack,  as  intermittent  fever;  while  all  diseases 
have  their  periods  of  exacerbation,  however  invariable  the 
symptoms  may  seem  to  be  to  the  ordinary  observer  or  even  to 
the  patient  himself. 

Doubtless  the  fact  that  certain  hereditary  diseases  do  not 
appear  in  the  offspring  at  once,  but  only  at  the  age  at  which 
they  were  manifested  in  the  parents,  is  owing  to  the  same 
cause. 

Let  us  now  examine  more  thoroughly  into  the  real  nature  of 
this  rhythm  which  prevades  the  entire  universe. 

If  a  bow  be  drawn  across  a  violin-string  on  which  some  small 
pieces  of  paper  have  been  placed,  these  will  be  seen  to  fly  off  ; 
and  if  the  largest  string  be  experimented  upon,  it  can  be  ob- 
served to  be  in  rapid  to-and-fro  motion,  known  as  vibration, 
which  motion  is  perfectly  regular,  a  definite  number  of  move- 
ments occurring  within  a  measured  period  of  time  ;  in  other 
words  the  motion  is  rhythmical.  In  strings  of  the  finest  size 
the  motion  is  not  visible,  but  we  judge  of  its  existence  because 
of  the  result,  which  is  in  each  instance  a  sound.  Sound  is  to  us, 
however,  an  affection  of  the  nerve  of  hearing  and  the  brain, 
owing  to  the  vibrations  of  the  ear  caused  by  similar  vibrations 
of  the  violin-strings.  The  movements  of  the  nerves  and  nerve- 
cells  are  invisible  and  molecular,  and  we  seem  to  be  justified  in 
regarding  molecular  movements  as  constant  and  associated 
with  all  the  properties  of  matter  whether  living  or  dead. 

We  see,  then,  that  all  things  living  and  lifeless  are  in  con- 
stant motion,  visible  or  invisible  ;  there  is  no  such  thing  in  the 
universe  as  stable  equilibrium.  Change,  ceaseless  change,  is 
written  on  all  things  ;  and,  so  far  as  we  can  judge,  these 
changes,  on  the  whole,  tend  to  higher  development.  Neither 
rhythm,  however,  nor  anything  else,  is  perfect.  Even  the  mo- 
tions of  planets  are  subject  to  perturbations  or  irregularities 
in  their  periodicity.  This  subject  is  plainly  boundless  in  its 
scope.  We  have  introduced  it  at  this  stage  to  prepare  for  its 
study  in  detail  in  dealing  with  each  function  of  the  animal 
body.  If  we  are  correct  as  to  the  universality  of  the  law  of 
rhythm,  its  importance  in  biology  deserves  fuller  recognition 
than  it  has  yet  received  in  works  on  physiology  ;  it  will,  ac- 
cordingly, be  frequently  referred  to  in  the  future  chapters  of 
this  book. 


GENERAL   BIOLOGY.  41 


THE  LAW   OF  HABIT. 

Every  one  must  have  observed  in  himself  and  others  the 
tendency  to  fall  into  set  ways  of  doing  certain  things,  in  which 
will  and  clear  pui'pose  do  not  come  prominently  into  view. 
Further  observation  shows  that  the  lower  animals  exhibit  this 
tendency,  so  that,  for  example,  the  habits  of  the  horse  or  the  dog 
may  be  an  amusing  reflection  of  those  of  the  master.  Trees  are 
seen  to  bend  permanently  in  the  direction  toward  which  the 
prevailing  winds  blow. 

The  violin  that  has  experienced  the  vibrations,  aroused  by 
some  master's  hand  acquires  a  potential  musical  capability  not 
possessed  by  an  instrument  equally  good  originally,  but  the 
molecular  movements  of  which  never  received  such  an  educa- 
tion. 

It  appears,  then,  that  underlying  what  we  call  habit,  there  is 
some  broad  law  not  confined  to  living  things  ;  indeed,  the  law 
of  habit  appears  to  be  closely  related  to  the  law  of  rhythm  we 
have  already  noticed.  Certain  it  is  that  it  is  inseparable  from 
all  biological  phenomena,  though  most  manifest  in  those  organ- 
isms provided  with  a  nervous  system,  and  in  that  system  itself. 
What  we  usually  call  habit,  however  expressed,  has  its  physical 
correlation  in  the  nervous  system.  We  may  refer  to  it  in  this 
connection  later:  but  the  subject  has  relations  so  numerous 
and  fundamental  that  it  seems  eminently  proper  to  introduce 
it  at  this  early  stage,  forming  as  it  does  one  of  those  corner- 
stones of  the  biological  building  on  which  the  superstructure 
must  rest. 

When  we  seek  to  come  to  a  final  explanation  of  habit  in  this 
case,  as  in  most  others,  in  which  the  fundamental  is  involved, 
we  are  soon  brought  against  a  wall  over  which  we  ai*e  unable 
to  climb,  and  through  which  no  light  comes  to  our  intellects. 

We  must  simply  believe,  as  the  result  of  observation,  that  it 
is  a  law  of  matter,  in  all  the  forms  manifested  to  us,  to  assume 
accustomed  modes  of  behavior,  perhaps  we  may  say  molecular 
movement,  in  obedience  to  inherent  tendencies.  But,  to  recog- 
nize this,  throws  a  flood  of  light  on  what  would  be  inexplicable, 
even  in  a  minor  degree.  We  can  not  explain  gravitation  in  it- 
self ;  but,  assuming  its  universality,  replaces  chaos  by  order  in 
our  speculations  on  matter. 

Turning  to  living  matter,  we  look  for  the  origin  of  habit  in 
the  apparently   universal    principle  that    primary  molecular 


42  COMPARATIVE  PHYSIOLOGY. 

movement  in  one  direction  renders  that  movement  easier  after- 
ward, and  in  proportion  to  the  frequency  of  repetition  ;  which 
is  equivalent  to  saying  that  functional  activity  facilitates  func- 
tional activity.  Once  accepting  this  as  of  universal  application 
in  biology,  we  have  an  explanation  of  the  origin,  the  compara- 
tive rigidity,  and  the  necessity  of  habit.  There  must  be  a  phys- 
ical basis  or  correlative  of  all  mental  and  moral  habits,  as  well 
as  those  that  may  be  manifested  during  sleep,  and  so  purely  in- 
dependent of  the  will  and  consciousness.  We  are  brought,  in 
fact,  to  the  habits  of  cells  in  considering  those  organs,  and  that 
combination  of  structures  which  makes  up  the  complex  individ- 
ual mammal.  It  is  further  apparent  that  if  the  cell  can  trans- 
mit its  nature  as  altered  by  its  experiences  at  all,  then  habits 
must  be  hereditary,  which  is  known  to  be  the  case. 

Instincts  seem  to  be  but  crystallized  habits,  the  inherited 
results  of  ages  of  functional  activity  in  certain  well-defined 
directions. 

To  a  being  with  a  highly  developed  moral  nature  like  man, 
the  law  of  habit  is  one  of  great,  even  fearful  significance.  We 
make  to-day  our  to-morrow,  and  in  the  present  we  are  deciding 
the  future  of  others,  as  our  present  has  been  made  for  us  in  part 
by  our  ancestors.  We  shall  not  pursue  the  subject,  which  is  of 
boundless  extent,  further  now,  but  these  somewhat  general 
statements  will  be  amplified  and  applied  in  future  chapters. 

THE   ORIGIN   OF   THE   FORMS   OF   LIFE. 

It  is  a  matter  of  common  observation  that  animals  originate 
from  like  kinds,  and  plants  from  forms  resembling  themselves  ; 
while  most  carefully  conducted  experiments  have  failed  to  show 
that  living  matter  can  under  any  circumstances  known  to  us 
arise  from  other  than  living  matter. 

That  in  a  former  condition  of  the  universe  such  may  have 
been  the  case  has  not  been  disproved,  and  seems  to  be  the  logical 
outcome  of  the  doctrine  of  evolution  as  applied  to  the  universe 
generally. 

By  evolution  is  meant  the  derivation  of  more  complex  and 
differentiated  forms  of  matter  from  simpler  and  more  homogene- 
ous ones.  When  this  theory  is  applied  to  organized  or  living 
forms,  it  is  termed  organic  evolution.  There  are  two  views  of 
the  origin  of  life  :  the  one,  that  each  distinct  group  of  plants 
and  animals  was  independently  created  ;  while  by  "  creation  "  is 


GENERAL  BIOLOGY.  43 

simply  meant  that  they  came  into  being  in  a  manner  we  know 
not  how,  in  obedience  to  the  will  of  a  First  Cause.  The  other 
view  is  denominated  the  theory  of  descent  with  modification, 
the  theory  of  transmutation,  organic  evolution,  etc.,  which 
teaches  that  all  the  various  forms  of  life  have  been  derived 
from  one  or  a  few  primordial  forms  in  harmony  with  the  recog- 
nized principles  of  heredity  and  variability.  The  most  widely 
known  and  most  favorably  received  exposition  of  this  theory  is 
that  of  Charles  Darwin,  so  that  his  views  will  be  first  presented 
in  the  form  of  a  hypothetical  case.  Assume  that  one  of  a  group 
of  living  forms  varies  from  its  fellows  in  some  particular,  and 
mating  with  another  that  has  similarly  varied,  leaves  progeny 
inheriting  this  characteristic  of  the  parents,  that  tends  to  be 
still  further  increased  and  rendered  permanent  by  successive 
pairing  with  forms  possessing  this  valuation  in  shape,  color,  or 
whatever  it  may  be.  We  may  suppose  that  the  variations  may 
be  numerous,  but  are  always  small  at  the  beginning.  Since  all 
animals  and  plants  tend  to  multiply  faster  than  the  means  of 
support,  a  competition  for  the  means  of  subsistence  arises,  in 
which  struggle  the  fittest,  as  judged  by  the  circumstances,  al- 
ways is  the  most  successful  ;  and  if  one  must  perish  outright,  it 
is  the  less  fit.  If  any  variation  arises  that  is  unfavorable  in 
this  contest,  it  will  render  the  possessor  a  weaker  competitor  : 
hence  it  follows  that  only  useful  variations  are  preserved.  The 
struggle  for  existence  is,  however,  not  alone  for  food,  but  for 
anything  which  may  be  an  advantage  to  its  possessor.  One  form 
of  the  contest  is  that  which  results  from  the  rivalry  of  members 
of  the  same  sex  for  the  possession  of  the  females  ;  and  as  the 
female  chooses  the  strongest,  most  beautiful,  most  active,  or  the 
supreme  in  some  respect,  it  follows  that  the  best  leave  the  great- 
est number  of  progeny.  This  has  been  termed  sexual  selection. 
In  determining  what  forms  shall  survive,  the  presence  of 
other  plants  or  animals  is  quite  as  important  as  the  abundance 
of  food  and  the  physical  conditions,  often  more  so.  To  illustrate 
this  by  an  example  :  Certain  kinds  of  clover  are  fertilized  by 
the  visits  of  the  bumble-bee  alone  ;  the  numbers  of  bees  exist- 
ing at  any  one  place  depends  on  the  abundance  of  the  field-mice 
which  destroy  the  nests  of  these  insects  ;  the  numbers  of  mice 
will  depend  on  the  abundance  of  creatures  that  prey  on  the 
mice,  as  hawks  and  owls  ;  these,  again,  on  the  creatures  that 
specially  destroy  them,  as  foxes,  etc. ;  and  so  on,  the  chain  of 
connections  becoming  more  and  more  lengthy. 


44 


COMPARATIVE   PHYSIOLOGY. 


Fig.  47.— Shows  the  embryos  of  four  mammals  in  the  three  eorresponding  stages  :  of 
a  hog  (II),  calf  (C),  rabbit  (R),  and  a  man  (M).  The  conditions  of  the  three  differ- 
ent stages  of  development,  which  the  three  cross-rows  (I,  II,  III)  represent,  are 
selected  to  correspond  as  exactly  as  possible.  The  first,  or  upper  cross-row,  I. 
represents  a  very  early  stage,  with  gill-openings,  and  without  limbs.  The  second 
(middle)  cross-row,  II,  shows  a  somewhat  later  stage,  with  the  first  rudiments  of 
limbs,  while  the  gill-openings  are  yet  retained.  The  third  (lowest)  cross-row,  III, 
shows  a  still  later  stage,  with  the  limbs  more  developed  and  the  gill-openings 


GENERAL   BIOLOGY.  45 

lost.  The  membranes  and  appendages  of  the  embryonic  body  (the  amnion,  yelk- 
sac,  allantois)  are  omitted.  The  whole  twelve  figures  are  slightly  magnified,  the 
upper  ones  more  than  the  lower.  To  facilitate  "the  comparison,  they  are  all  re- 
duced to  nearly  the  same  size  in  the  cuts.  All  the  embryos  are  seen  from  the  left 
side  ;  the  head  extremity  is  above,  the  tail  extremity  below  ;  the  arched  back 
turned  to  the  right,  The  letters  indicate  the  same  parts  in  all  the  twelve  figures, 
namely:  v,  fore-brain;  z,  twist-brain;  m,  mid-brain;  h,  hind-brain;  re,  after-brain; 
r,  spinal  marrow-  e,  nose;  «,  eye;  o,  ear;  k,  gill-arches;  g.  heart;  iv,  vertebral 
column;  /,  fore-limbs;  b,  hind-limbs;  s,  tail.    (After  Haeckel.) 

If  a  certain  proportion  of  forms  varying  similarly  were  sep- 
arated by  any  great  natural  barrier,  as  a  chain  of  lofty  mountains 
or  an  intervening  body  of  water  of  considerable  extent,  and  so  pre- 
vented from  breeding  with  forms  that  did  not  vary,  it  is  clear  that 
-there  would  be  greater  likelihood  of  their  differences  being  pre- 
served and  augmented  up  to  the  point  of  their  greatest  usefulness. 

We  may  now  inquire  whether  such  has  actually  been  the 
course  of  events  in  nature.  The  evidence  may  be  arranged  un- 
der the  following  heads  : 

1.  Morphology. — Briefly,  there  is  much  that  is  common  to 
entire  large  groups  of  animals  ;  so  great,  indeed,  are  the  resem- 
blances throughout  the  whole  animal  kingdom  that  herein  is 
found  the  strongest  argument  of  all  for  the  doctrine  of  descent. 
To  illustrate  by  a  single  instance — fishes,  reptiles,  birds,  and 
mammals  possess  in  common  a  vertebral  column  bearing  the 
same  relationship  to  other  parts  of  the  animal.  It  is  because  of 
resemblances  of  this  kind,  as  well  as  by  their  differences,  that 
naturalists  are  enabled  to  classify  animals. 

2.  Embryology.— In  the  stages  through  which  animals  pass 
in  their  development  from  the  ovum  to  the  adult,  it  is  to  be  ob- 
served that  the  closer  the  resemblance  of  the  mature  organism 
in  different  groups,  the  more  the  embryos  resemble  one  another. 
Up  to  a  certain  stage  of  development  the  similarity  between 
groups  of  animals,  widely  separated  in  their  post-embryonic 
life,  is  marked  :  thus  the  embryo  of  a  reptile,  a  bird,  and  a  mam- 
mal have  much  in  common  in  their  earlier  stages.  The  embryo 
of  the  mammal  passes  through  stages  which  represent  condi- 
tions which  are  permanent  in  lower  groups  of  animals,  as  for 
example  that  of  the  branchial  arches,  which  are  represented  by 
the  gills  in  fishes.  It  may  be  said  that  the  developmental  his- 
tory of  the  individual  (ontogeny)  is  a  brief  recapitulation  of  the 
development  of  the  species  (phylogeny).  Apart  from  the  theory 
of  descent,  it  does  not  seem  possible  to  gather  the  true  signifi- 
cance of  such  facts,  which  will  become  plainer  after  the  study 
of  the  chapters  on  reproduction. 

3.  Mimicry  may  be  cited  as  an  instance  of  useful  adaptation. 


46  COMPARATIVE   PHYSIOLOGY. 

Thus,  certain  beetles  resemble  bees  and  wasps,  which  latter  are 
protected  by  stings.  It  is  believed  that  such  groups  of  beetles 
as  these  arose  by  a  species  of  selection  ;  those  escaping  enemies 
which  chanced  to  resemble  dreaded  insects  most,  so  that  birds 
which  were  accustomed  to  prey  on  beetles,  yet  feared  bees,  would 
likewise  avoid  the  mimicking  forms. 

4.  Rudimentary  Organs. — Organs  which  were  once  func- 
tional in  a  more  ancient  form,  but  serve  no  use  in  the  creatures 
in  which  they  are  now  found,  have  reached,  it  is  thought,  their 
rudimentary  condition  through  long  periods  of  comparative 
disuse,  in  many  generations.  Such  are  the  rudimentary  mus- 
cles of  the  ears  of  man,  or  the  undeveloped  incisor  teeth  found 
in  the  upper  jaw  of  ruminants. 

5.  Geographical  Distribution.— It  can  not  be  said  that  ani- 
mals and  plants  are  always  found  in .  the  localities  where  they 
are  best  fitted  to  flourish.  This  has  been  well  illustrated  within 
the  lifetime  of  the  present  generation,  for  the  animals  intro- 
duced into  Australia  have  many  of  them  so  multiplied  as  to 
displace  the  forms  native  to  that  country.  But,  if  we  assume 
that  migrations  of  animals  and  transmutations  of  species  have 
taken  place,  this  difficulty  is  in  great  part  removed. 

6.  Paleontology. — The  rocks  bear  record  to  the  former  exist- 
ence of  a  succession  of  related  forms;  and,  though  all  the  in- 
termediate links  that  probably  existed  have  not  been  found, 
the  apparent  discrepancy  can  be  explained  by  the  nature  of 
the  circumstances  under  which  fossil  forms  are  preserved  ;  and 
the  "imperfection  of  the  geological  record." 

It  is  only  in  the  sedimentary  rocks  arising  from  mud  that 
fossils  can  be  preserved,  and  those  animals  alone  with  hard 
parts  are  likely  to  leave  a  trace  behind  them ;  while  if  these 
sedimentary  rocks  with  their  inclosed  fossils  should,  owing  to 
enormous  pressure  or  heat  be  greatly  changed  (metamorphosed), 
all  trace  of  fossils  must  disappear — so  that  the  earliest  forms 
of  life,  those  that  would  most  naturally,  if  preserved  at  all,  be 
found  in  the  most  ancient  rocks,  are  wanting,  in  consequence 
of  the  metamorphism  which  such  formations  have  undergone. 
Moreover,  our  knowledge  of  the  animal  remains  in  the  earth's 
crust  is  as  yet  very  incomplete,  though,  the  more  it  is  explored, 
the  more  the  evidence  gathers  force  in  favor  of  organic  evolu- 
tion. But  it  must  be  remembered  that  those  groups  constitut- 
ing species  are  in  geological  time  intermediate  links. 

7.  Fossil  and  Existing  Species.— If  the  animals  and  plants 


GENERAL  BIOLOGY. 


47 


now  peopling  the  earth  were  entirely  different  from  those  that 
flourished  in  the  past,  the  objections  to  the  doctrine  of  descent 
would  be  greatly  strengthened ;  but  when  it  is  found  that  there 
is  in  some  cases  a  scarcely  broken  succession  of  forms,  great 
force  is  added  to  the  arguments  by  which  we  are  led  to  infer 
the  connection  of  all  forms  with  one  another. 

To  illustrate   by  a   single  instance:  the   existing   group  of 
horses,  with  a  single  toe  to  each  foot,  was  preceded  in  geological 


Fig.  48. — Bones  of  the  feet  of  the  different  genera  of  Equidm  (after  Marsh),  a,  foot 
of  Orohippus  (Eocene);  b,  foot  of  Anchitlierium  (Lower  Miocene);  c,  foot  of  Hip- 
parion  (Pliocene);  d,  foot  of  the  recent  genus  Equus. 

time  in  America  by  forms  with  a  greater  number  of  toes,  the 
latter  increasing  according  to  the  antiquity  of  the  group. 
These  forms  occur  in  succeeding  geological  formations.  It  is 
impossible  to  resist  the  conclusion  that  they  are  related  gene- 
alogically (phylogenetically). 

8.  Progression. — Inasmuch  as  any  form  of  specialization  that 
would  give  an  animal  or  plant  an  advantage  in  the  struggle  for 
existence  would  be  preserved,  and  as  in  most  cases  when  the 
competing  forms  are  numerous  such  would  be  the  case,  it  is 
possible  to  understand  how  the  organisms  that  have  appeared 
have  tended,  on  the  whole,  toward  a  most  pronounced  pro- 
gression in  the  scale  of  existence.  This  is  well  illustrated  in 
the  history  of  civilization.  Barbarous  tribes  give  way  before 
civilized  man  with  the  numberless  subdivisions  of  labor  he  in- 
stitutes in  the  social  organism.  It  enables  greater  numbers  to 
flourish,  as  the  competition  is  not  so  keen  as  if  activities  could 
be  exercised  in  a  few  directions  only. 

9.  Domesticated  Animals. — Darwin  studied  our  domestic  ani- 
mals long  and  carefully,  and  drew  many  important  conclusions 


4S 


COMPARATIVE   PHYSIOLOGY. 


from  his  researches.  He  was  convinced  that  they  had  all  been 
derived  from  a  few  wild  representatives,  in  accordance  with  the 
principles  of  natural  selection.  Breeders  have  both  consciously 
and  unconsciously,  formed  races  of  animals  from  stocks  which 
the  new  groups  have  now  supplanted;  while  primitive  man 
had  tamed  various  species  which  he  kept  for  food  and  to  assist 
in  the  chase,  or  as  beasts  of  burden.  It  is  impossible  to  believe 
that  all  the  different  races  of  dogs  have  originated  from  dis- 
tinct wild  stocks,  for  many  of  them  have  been  formed  within 
recent  periods ;  in  fact,  it  is  likely  that  to  the  jackal,  wolf,  and 
fox,  must  we  look  for  the  wild  progenitors  of  our  dogs.  Dar- 
win concluded  that,  as  man  had  only  utilized  the  materials 
Nature  provided  in  forming  his  races  of  domestic  animals,  he 
had  availed  himself  of  the  variations  that  arose  spontaneously, 
and  increased  and  fixed  them  by  breeding  those  possessing  the 
same  variation  together,  so  the  like  had  occurred  without  his 
aid  in  nature  among  wild  forms. 

Evolutionists  are  divided  as  to  the  origin  of  man  himself  ; 
some,  like  Wallace,  who  are  in  accord  with  Darwin  as  to  the 


4-    3 


Fia.  40.— Skeleton  of  hand  or  fore-foot  of  six  mammals.  I.  man;  II,  dog;  III,  pit;; 
IV,  ox;  V,  tapir;  VI,  horse,  r,  radius;  u,  ulna;  a,  scaphoid;  b,  semi-lunur;  c, 
Iriquetrum  (cuneiform);  d,  trapezium;  e,  trapezoid;  /,  capitatum  (unciform  pro- 
cess); /•/,  hamatum  (unciform  bone);  p,  pisiform;  ij  thumb;  2,  digit;  3,  middle 
finger;  4,  ring-finger;  5,  little  finger.    (After  Gegenbaur.) 


origin  of   living  forms  in   general,  believe  that  the  theory  of 
natural  selection  does  not  suffice  to  account  for  the  intellectual 


GENERAL   BIOLOGY. 


49 


50  COMPARATIVE   PHYSIOLOGY. 

and  moral  nature  of  man.  Wallace  believes  that  man's  body 
has  been  derived  from  lower  forms,  but  that  his  higher  nature 
is  the  result  of  some  unknown  law  of  accelerated  development ; 
while  Darwin,  and  those  of  his  way  of  thinking,  consider  that 
mau  in  his  entire  nature  is  but  a  grand  development  of  powers 
existing  in  minor  degree  in  the  animals  below  him  in  the  scale. 
Summary. — Every  group  of  animals  and  plants  tends  to  in- 
crease in  numbers  in  a  geometrical  progression,  and  must,  if 
unchecked,  overrun  the  earth.  Every  variety  of  animals  and 
plants  imparts  to  its  offspring  a  general  resemblance  to  itself, 
but  with  minute  variations  from  the  original.  The  variations 
of  offsprings  may  be  in  any  direction,  and  by  accumulation 
constitute  fixed  differences  by  which  a  new  group  is  marked 
off.  In  the  determination  of  the  variations  that  persist,  the  law 
of  survival  of  the  fittest  operates. 


REPRODUCTION. 


As  has  been  already  noticed,  protoplasm,  in  whatever  form, 
after  passing  through  certain  stages  in  development,  undergoes 
a  decline,  and  finally  dies  and  joins  the  world  of  unorganized 
matter  ;  so  that  the  permanence  of  living  things  demands  the 
constant  formation  of  new  individuals.  Groups  of  animals 
and  plants  from  time  to  time  become  extinct ;  but  the  lifetime 
of  the  species  is  always  long  compared  with  that  of  the  indi- 
vidual. Reproduction  by  division  seems  to  arise  from  an  exi- 
gency of  a  nutritive  kind,  best  exemplified  in  the  simpler  or- 
ganisms. When  the  total  mass  becomes  too  great  to  be  supported 
by  absorption  of  pabulum  from  without  by  the  surface  of  the 
body,  division  of  the  organism  must  take  place,  or  death  ensues. 
It  appears  to  be  a  matter  of  indifference  how  this  is  accom- 
plished, whether  by  fission,  endogenous  division,  or  gemmation, 
so  long  as  separate  portions  of  protoplasm  result,  capable  of 
leading  an  independent  existence.  The  very  undifferentiated 
character  of  these  simple  forms  prepares  us  to  understand  how 
each  fragment  may  go  through  the  same  cycle  of  changes  as 
the  parent  form.  In  such  cases,  speaking  generally,  a  million 
individuals  tell  the  same  biological  story  as  one  ;  yet  these 
must  exist  as  individuals,  if  at  all,  and  not  in  one  great  united 
mass.  But  in  the  case  of  conjugation,  which  takes  place  some- 
times in  the  same  groups  as  also  multiply  by  division  in  its 
various  forms,  there  is  plainly  an  entirely  new  aspect  of  the 
case  presented.  We  have  already  shown  that  no  two  cells,  how- 
ever much  alike  they  may  seem  as  regards  form  and  the  cir- 
cumstances under  which  they  exist,  can  have,  in  the  nature  of 
the  case,  precisely  the  same  history,  or  be  the  subjects  of  ex- 
actly the  same  experiences.  We  have  also  pointed  out  that  all 
these  phenomena  of  cell-life  are  known  to  us  only  as  adapta- 
tions of  internal  to  external  conditions ;  for,  though  we  may  not 
be  always  able  to  trace  this  connection,  the  inference  is  justi- 


52  COMPARATIVE   PHYSIOLOGY. 

Sable,  because  there  are  no  facts  known  to  xis  that  contradict 
such  an  assumption,  while  those  that  are  within  our  knowledge 
bear  out  the  generalization.  We  have  already  learned  that  liv- 
ing things  are  in  a  state  of  constant  change,  as  indeed  are  all 
things  ;  we  have  observed  a  constant  relation  between  certain 
changes  in  the  environment,  or  sum  total  of  the  surrounding 
conditions,  as,  for  example,  temperature,  and  the  behavior  of 
the  protoplasm  of  plants  and  animals ;  so  that  we  must  believe 
that  any  one  form  of  protoplasm,  however  like  another  it  may 
seem  to  our  comparatively  imperfect  observation,  is  different 
in  some  respects  from  every  other — as  different,  relatively,  as 
two  human  beings  living  in  the  same  community  during  the 
whole  of  their  lives ;  and  in  many  cases  as  unlike  as  individuals 
of  very  different  nationality  and  history.  "We  are  aware  that 
when  two  such  persons  meet,  provided  the  unlikeness  is  not  so 
great  as  to  prevent  social  intercourse,  intercommunication  may 
prove  very  instructive.  Indeed,  the  latter  grows  out  of  the 
former;  our  illustration  is  itself  explained  by  the  law  we  are 
endeavoring  to  make  plain.  It  would  appear,  then,  that  con- 
tinuous division  of  protoplasm  without  external  aid  is  not  pos- 
sible ;  but  that  the  vigor  necessary  for  this  must  in  some  way 
be  imparted  by  a  particle  (cell)  of  similar,  yet  not  wholly  like, 
protoplasm.  This  seems  to  furnish  an  explanation  of  the  neces- 
sity for  the  conjugation  of  living  forms,  and  the  differentiation 
of  sex.  Very  frequently  conjugation  in  the  lowest  animals  and 
plants  is  followed  by  long  periods  when  division  is  the  prevail- 
ing method  of  reproduction.  It  is  worthy  of  note,  too,  that 
when  living  forms  conjugate,  they  both  become  quiescent  for  a 
longer  or  shorter  time.  It  is  as  though  a  period  of  preparation 
preceded  one  of  extraordinary  activity.  We  can  at  present 
trace  only  a  few  of  the  steps  in  this  rejuvenation  of  life-stuff. 
Some  of  these  have  been  already  indicated,  which,  with  others, 
will  now  be  further  studied  in  this  division  of  our  subject,  both 
because  reproduction  throws  so  much  light  on  cell-life,  and  be- 
cause it  is  so  important  for  the  understanding  of  the  physio- 
logical behavior  of  tissues  and  organs.  It  may  be  said  to  be 
quite  as  important  that  the  ancestral  history  of  the  cells  of  an 
organism  be  known  as  the  history  of  the  units  composing  a 
community.  A,  B,  and  C,  can  be  much  better  understood  if 
we  know  something  alike  of  the  history  of  their  race,  their  an- 
cestors, and  their  own  past;  so  is  it  with  the  study  of  any  indi- 
vidual animal,  or  group  of  animals  or  plants.      Accordingly, 


REPRODUCTION.  53 

embryology,  or  the  history  of  the  origin  and  development  of 
tissues  and  organs,  will  occupy  a  prominent  place  in  the  vari- 
ous chapters  of  this  work.  The  student  will,  therefore,  at  the 
outset  be  furnished  with  a  general  account  of  the  subject,  while 
many  details  and  applications  of  principles  will  be  left  for  the 
chapters  that  treat  of  the  functions  of  the  various  organs  of 
animals.  The  more  knowledge  the  student  possesses  of  zoology 
the  better,  while  this  science  will  appear  in  a  new  light  under 
the  study  of  embryology. 

Animals  are  divisible,  according  to  general  structure,  into 
Protozoa,  or  unicellular  animals,  and  Metazoa,  or  multicellular 
forms — that  is,  animals  composed  of  cell  aggregates,  tissues,  or 
organs.  Among  the  latter  one  form  of  reproduction  appears 
for  the  first  time  in  the  animal  kingdom,  and  becomes  all  but 
universal,  though  it  is  not  the  exclusive  method  ;  for,  as  seen  in 
Hydra,  both  this  form  of  generation  and  the  more  primitive 
gemmation  occur.  It  is  known  as  sexual  multiplication,  which 
usually,  though  not  invariably,  involves  conjugation  of  two  un- 
like cells  which  may  arise  in  the  same  or  different  individuals. 
That  these  cells,  known  as  the  male  and  female  elements,  the 
ovum  and  the  spermatozoon,  are  not  necessarily  radically  dif- 
ferent, is  clear  from  the  fact  that  they  may  arise  in  the  one  in- 
dividual from  the  same  tissue  and  be  mingled  together.  These 
cells,  however,  like  all  others,  tell  a  story  of  continual  progress- 
ive differentiation  corresponding  to  the  advancing  evolution  of 
higher  from  lower  forms.  Thus  hermaphroditism,  or  the  coex- 
istence of  organs  for  the  production  of  male  and  of  female  cells 
in  the  same  individual,  is  confined  to  invertebrates,  among 
which  it  is  rather  the  exception  than  the  rule.  Moreover,  in 
such  hermaphrodite  forms  the  union  of  cells  with  greater  differ- 
ence in  experiences  is  provided  for  by  the  union  of  different  in- 
dividuals, so  that  commonly  the  male  cell  of  one  individual 
unites  with  (fertilizes)  the  female  cell  of  a  different  individual. 
It  sometimes  happens  that  among  the  invertebrates  the  cells 
produced  in  the  female  organs  of  generation  possess  the  power 
of  division,  and  continued  development  wholly  independently 
of  the  access  of  any  male  cell  {parthenogenesis)  ;  such,  how- 
ever, is  almost  never  the  exclusive  method  of  increase  for  any 
group  of  animals,  and  is  to  be  regarded  as  a  retention  of  a  more 
ancient  method,  or  perhaps  rather  a  reversion  to  a  past  biologi- 
cal condition.  No  instance  of  complete  parthenogenesis  is 
known  among  vertebrates,  although  in  birds  partial  develop- 


54  COMPARATIVE   PHYSIOLOGY. 

merit  of  the  egg  may  take  place  independently  of  the  influence 
of  the  male  sex.  The  best  examples  of  parthenogenesis  are  to 
be  found  among  insects  and  crustaceans. 

It  is  to  be  remembered  that,  while  the  cells  which  form  tbe 
tissues  of  the  body  of  an  animal  have  become  specialized  to 
discharge  one  particular  function,  they  have  not  wholly  lost 
all  others  ;  they  do  not  remain  characteristic  amceboids,  as  we 
may  term  cells  closely  resembling  Amoeba  in  behavior,  nor  do 
they  wholly  forsake  their  ancestral  habits.  They  all  retain  the 
power  of  reproduction  by  division,  especially  when  young  and 
most  vigorous  ;  for  tissues  grow  chiefly  by  the  production  of 
new  cells  rather  than  the  enlargement  of  already  mature  ones. 
Cells  wear  out  and  must  be  replaced,  which  is  effected  by  the 
processes  already  described  for  Amoeba  and  similar  forms. 
Moreover,  there  is  retained  in  the  blood  of  animals  an  army  of 
cells,  true  amceboids,  ever  ready  to  hasten  to  repair  tissues  lost 
by  injury.  These  are  true  remnants  of  an  embryonic  condition  ; 
for  at  one  period  all  the  cells  of  the  organism  were  of  this  un- 
differentiated, plastic  character.  But  the  cell  {ovum)  from 
which  the  individual  in  its  entirety  and  with  all  its  complexity 
arises  mostly  by  the  union  with  another  cell  (spermatozoon), 
must  be  considered  as  one  that  has  remained  unspecialized 
and  retained,  and  perhaps  increased  its  reproductive  functions. 
They  certainly  have  become  more  complex.  The  germ-cell 
may  be  considered  unspecialized  as  regards  other  functions,  but 
highly  specialized  in  the  one  direction  of  exceedingly  great  ca- 
pacity for  growth  and  complex  division,  if  we  take  into  account 
the  whole  chain  of  results  ;  though  in  considering  this  it  must 
be  borne  in  mind  that  after  a  certain  stage  of  division  each 
individual  cell  repeats  its  ancestral  history  again  ;  that  is  to 
say,  it  divides  and  gives  rise  to  cells  which  progress  in  turn  as 
well  as  multiply.  From  another  point  of  view  the  ovum  is  a 
marvelous  storehouse  of  energy,  latent  or  potential,  of  course, 
but  under  proper  conditions  liberated  in  varied  and  unexpected 
forms  of  force.  It  is  a  sort  of  reservoir  of  biological  energy 
in  the  most  concentrated  form,  the  liberation  of  which  in  sim- 
pler forms  gives  rise  to  that  complicated  chain  of  events  which 
is  termed  by  the  biologist  development,  but  which  may  be  ex- 
pressed by  the  physiologist  as  the  transformation  of  potential 
into  kinetic  energy,  or  the  energy  of  motion.  Viewed  chemi- 
cally, it  is  the  oft-repeated  story  of  the  production  of  forms,  of 
greater  stability  and  simplicity,  from  more  unstable  and  com- 


REPRODUCTION. 


55 


plex  ones,  involving  throughout  the  process  of  oxidation  ;  for 
it  niust  ever  be  kept  in  mind  that  life  and  oxidation  are  con- 
comitant and  inseparable.  The  further  study  of  reproduction 
in  the  concrete  will  render  the  meaning  and  force  of  many  of 
the  above  statements  clearer. 


THE   OVUM, 

The  typical  female  cell,  or  ovum,  consists  of  a  mass  of  pro- 
toplasm, usually  globular  in  form,  containing  a  nucleus  and 
nucleolus. 

The  ovum  may  or  may  not  be  invested  by  a  membrane  ;  the 
protoplasm  of  the  body  of  the  cell  is  usually  highly  granular, 
and  may  have  stored  up  within  it  a  varying  amount  of  proteid 
material  (food-yelk),  which  has  led  to  division  of  ova  into 
classes,  according  to  the  manner  of  distribution  of  this  nutri- 
tive reserve.  It  is  either  concentrated  at  one  pole  (telolecith- 
al) ;  toward  the  center  (centrolecithal) ;  or  evenly  distributed 
throughout  (alecithal). 
During  development    this  "f7 

material  is  converted  by  x^ 
the  agency  of  the  cells  of 
the  young  organism  (em- 
bryo) into  active  proto- 
plasm ;  in  a  word,  they 
feed  upon  and  assimilate 
or  build  up  this  food-stuff 
into  their  own  substance, 
as  Amoeba  does  with  any 
proteid  material  it  appro- 
priates. 

The  nucleus  (germinal 
vesicle)  is  large  and  well 
defined,  and  contains  with- 
in itself  a  highly  refractive 
nucleolus  (germinal  spot). 
These  closely  resemble  in  general  the  rest  of  the  cell,  but  stain 
more  deeply  and  are  chemically  different  in  that  they  contain 
nucleine  (nucleoplasm,  chromatin  >. 

It  will  be  observed  that  the  ovum  differs  in  no  essential  par- 
ticular of  structure  from  other  cells.  Its  differences  are  hidden 
ones  of  molecular  structure  and  functional  behavior.      In  ac- 


Fig.  55. — Semi-diagrammatic  representation  of 
a  mammalian  ovum  (Sch&fer).  Highly  mag- 
nified. z}>,  zonapellucida;  in,  vitellus  (yelk); 
gv,  germinal  vesicle;  gs,  germinal  spot. 


56 


COMPARATIVE   PH  YSIOLOG Y, 


cordance  with  the  diverse  circumstances  under  which  ova  ma- 
ture and  develop,  certain  variations  in  structure,  mostly  of  the 
nature  of  additions,  present  themselves. 

Thus,  ova  may  be  naked,  or  provided  with  one  or  more  cover- 
ings. In  vertebrates  there  are  usually  two  membranes  around 
the  protoplasm  of  the  ovum  :  a  delicate  covering  (Vitelline 
membrane)  beneath  which  there  is  another,  which  is  sieve-like 
from  numerous  perforations  (zona  radiata,  or  z.  pellucida). 
The  egg  membrane  may  be  impregnated  with  lime  salts  (shell). 
Between  the  membranes  and  the  yelk  there  is  a  fluid  albumi- 
nous substance  secreted  by  the  glands  of  the  oviduct,  or  by  other 
special  glands,  which  provide  proteid  nutriment  in  different 
physical  condition  from  that  of  the  yelk. 

The  general  naked-eye  appearances  of  the  ovum  may  be 
learned  from  the  examination  of  a  hen's  egg,  which  is  one  of 

11 

wy- 

yy 


eh. I 


Fig.  56. — Diagrammatic  section  of  an  unimpregnated  fowl's  egg  (Foster  and  Balfour, 
after  Allen  Thomson),  bl,  blastoderm  or  cicatricnla;  10.  y,  white  yelk;  y.  y,  yel- 
low yelk;  ch.l.  chalaza;  i.s.m,  inner  layer  of  shell  membrane;  s.  m,  outer  layer 
of  shell  membrane;  s.  shell;  a.  c.  h,  air-space:  w,  the  white  of  the  egg;  v.  (,  vitel- 
line membrane  ;  x,  the  denser  albuminous  layer  lying  next  the  vitelline  mem- 
brane. 

the  most  complicated  known,  inasmuch  as  it  is  adapted  for 
development  outside  of  the  body  of  the  mother,  and  must,  con- 
sequently, be  capable  of  preserving  its  form  and  essential  vital 
properties  in  a  medium  in  which  it  is  liable  to  undergo  loss  of 
water,  protected  as  it  now  is  with  shell,  etc.,  but  which,  at  the 


REPRODUCTION.  57 

same  time  permits  the  entrance  of  oxygen  and  moisture,  and 
conducts  heat,  all  being-  essential  for  the  development  of  the 
germ  within  this  large  food-mass.  The  shell  serves,  evidently, 
chiefly  for  protection,  since  the  eggs  of  serpents  (snakes,  turtles, 
etc.)  are  provided  only  with  a  very  tough  membranous  cover- 
ing, this  answering  every  purpose  in  eggs  buried  in  sand  or 
otherwise  protected  as  theirs  usually  are.  As  the  hen's  egg  is 
that  most  readily  studied  and  most  familiar,  it  may  be  well  to 
describe  it  in  somewhat  further  detail,  as  illustrated  in  the 
above  figure,  from  the  examination  of  which  it  will  be  ap- 
parent that  the  yelk  itself  is  made  up  of  a  white  and  yellow 
portion  distributed  in  alternating  zones,  and  composed  of  cells 
of  different  microscopical  appearances.  The  clear  albumen  is 
structureless. 

The  relative  distribution,  and  the  nature  of  the  accessory  or 
non-essential  parts  of  the  hen's  egg,  will  be  understood  when  it 
is  remembered  that,  after  leaving  its  seat  of  origin,  which  will 
be  presently  described,  the  ovum  passes  along  a  tube  (oviduct) 
by  a  movement  imparted  to  it  by  the  muscular  walls  of  the 
latter,  similar  to  that  of  the  gullet  during  the  swallowing  of 
food  ;  that  this  tube  is  provided  with  glands  which  secrete  in 
turn  the  albumen,  the  membrane  (outer),  the  lime  salts  of  the 
shell,  etc.  The  twisted  appearance  of  the  rope-like  structures 
(chalazce)  at  each  end  is  owing  to  the  spiral  rotatory  movement 
the  egg  has  undergone  in  its  descent. 

The  air-chamber  at  the  larger  end  is  not  present  from  the 
first,  but  results  from  evaporation  of  the  fluids  of  the  albumen 
and  the  entrance  of  atmospheric  air  after  the  egg  has  been  laid 
some  time. 


THE   ORIGIN  AND   DEVELOPMENT   OF   THE    OVUM. 

Between  that  protrusion  of  cells  which  gives  rise  to  the  bud 
which  develops  directly  into  the  new  individual,  and  that  which 
forms  the  ovary  within  which  the  ovum  as  a  modified  cell  arises, 
there  is  not  in  Hydra  much  difference  at  first  to  be  observed. 

In  the  mammal,  however,  the  ovary  is  a  more  complex  struct- 
ure, though,  relatively  to  many  organs,  still  simple.  It  consists, 
n  the  main,  of  connective  tissue  supplied  with  vessels  and  nerves 
inclosing  modifications  of  that  tissue  (Graafian  follicles)  within 
which  the  ovum  is  matured.  The  ovum  and  the  follicles  arise 
from  an  inversion  of  epithelial  cells,  on  a  portion  of  the  body 


58 


COMPARATIVE  PHYSIOLOGY. 


cavity  (germinal  ridge),  which  give  rise  to  the  oviim  itself,  and 
the  other  cells  surrounding  it  in  the  Graafian  follicle.     At  first 

these  inversions  form 
tubules  (egg-tubes)  which 
latter  become  broken  up 
into  isolated  nests  of 
cells,  the  forerunners  of 
the  Graafian  follicles. 

The  Graafian  follicle 
consists  externally  of  a 
fibrous  capsule  (tunica 
fibrosa),  in  close  relation 
to  which  is  a  layer  of  cap- 
illary blood-vessels  (tu- 
nica vasculosa),  the  two 
together  forming  the  gen- 
eral covering  (tunica 
propria)  for  the  more 
delicate  and  important 
cells  within.     Lining  the 

Fig.  57. — Section  through  portion  of  the  ovary  of  ,        .     .         n                „            .. 

mammal,  illustrating  mode  of  development  of  tunic  IS  a  layer  Of    small, 

the  Graafian  follicles  (Wiedersheim).    D,  dis-  "„__^,__-i,„+     „,,-k:„„i     „„iio, 

cue  proligerus  ;   El,  ripe  ovum;   G,  follicular  SOmewnat    CUDlcai     cells 

cells  of  germinal  epithelium;  g,  blood-vessels;  /anovnhvrivtri     rtvnv><)i7n<zri} 

K,  germinal  vesicle  (nucleus)  and  germinal  Wiemorana     granulosa), 

spot  (nucleolus) ;    KE,  germinal  epithelium;  which  at  One  part  invest 
Lf,  liquor  foil iculi;  Mg,  membrana  or  tunica 

granulosa,  or  follicular  epithelium;  Mp,  zona  the  OVUm  Several  layers 

pellucida ;  PS,  ingrowths  from  the  germinal  <.           ,  7.                    T  •             x 

of  which   deep  (discus  proligerus), 


epithelium,  ovarian  tubes,  by  means 
some  of  the  nests  retain  their  connection  with 
the  epithelium;  S.  cavity  which  appears  with- 
in the  Graafian  follicle;  So,  stroma  of  ovary; 
Tf,  theca  folliculi  or  capsule  ;  U,  primitive 
ova.  When  an  ovum  with  its  surrounding 
cells  has  become  separated  from  a  nest,  it  is 
known  as  a  Graafian  follicle. 


while  the  remainder  of 
the  space  is  filled  by  a 
fluid  (liquor  folliculi) 
probably  either  secreted 
by  the  cells  themselves, 
or  resulting  from  the  disintegration  of  some  of  them,  or  both. 

In  viewing  a  section  of  the  ovary  taken  from  a  mammal  at 
the  breeding-season,  ova  and  Graafian  follicles  may  be  seen  in 
all  stages  of  development — those,  as  a  rule,  nearest  the  surface 
being  the  least  matured.  The  Graafian  follicle  appears  to  pass 
inward,  to  undergo  growth  and  development  and  again  retire 
toward  the  exterior,  where  it  bursts,  freeing  the  ovum,  which  is 
conducted  to  the  site  of  its  future  development  by  appropriate 
mechanism  to  be  described  hereafter. 

Changes  in  the  Ovum  itself.— The  series  of  transformations 
that  take  place  in  the  ovum  before  and  immediately  after  the 


REPRODUCTION. 


59 


access  of  the  male  element  is,  in  the  opinion  of  many  biologists, 
of  the  highest  significance,  as  indicating  the  course  evolution 


m 


'i-r-'-^ZX?       Sill 


Mw/M 


Fig.  58.— Sagittal  section  of  the  ovary  of  an  adult  bitch  (after  Waldeyerl.  o.  e,  ova- 
rian epithelium;  o.  t,  ovarian  tubes;  y.f,  younger  follicles;  o.f,  older  follicle; 
d.  v,  discus  proligerus,  with  the  ovum;  e,  epithelium  of  a  second  ovum  in  the  same 
follicle:  f.  c,  fibrous  coat  of  the  follicle;  p.  c,  proper  coat  of  the  follicle;  e.  f.  epi- 
thelium.of  the  follicle  (membrana  granulosa);  a./,  collapsed  atrophied  follicle; 
b.  /'.blood-vessels;  c.  t,  cell-tubes  of  the  parovarium  divided  longitudinally  and 
transversely;  /.(/.tubular  depression  of  the  ovarian  epithelium  in  the  tissue  of 
the  ovary;  b.  e,  beginning  of  the  ovarian  epithelium,  close  to  the  lower  border  of 
the  ovary. 


has  followed  in  the  animal  kingdom,  as  well  as  instructive  in 
illustrating  the  behavior  of  nuclei  generally. 


60 


COMPARATIVE   PHYSIOLOGY. 


The  germinal  vesicle  may  acquire  powers  of  slow  movement 
(amoeboid),  and  the  germinal  spot  disappear  :  the  former  passes 
to  one  surface  (pole)  of  the  ovum  ;  both  these  structures  may 
undergo  that  peculiar  form  of  rearrangement  (karyokinesis) 
which  may  occur  in  the  nuclei  and  nucleoli  of  other  cells  prior 
to  division  ;  in  other  words,  the  ovum  has  features  common  to  it 
and  many  other  cells  in  that  early  stage  which  precedes  the  com- 
plicated transformations  which  constitute  the  future  history  of 
the  ovum. 

A  portion  of  the  changed  nucleus  (aster)  with  some  of  the 
protoplasm  of  the  cell  accumulates  at  one  surface  (pole),  which 
is  termed  the  upper  pole  because  it  is  at  this  region  that  the  epithe- 
lial cells  will  be  iiltimately  developed,  and  is  separated.  This  pro- 
cess is  repeated.     These  bodies  (polar  cells,  polar  globules,  etc.), 


Fig.  59.— Formation  of  polar  cells  in  a  star-fish  (Asterias  glacialis)  (from  Geddes, 
A — K  after  Fol,  L  after  O.  Hertwig).  A,  ripe  ovum  with  eccentric  germinal  vesi- 
cle and  spot;  B — D.  gradual  metamorphosis  of  germinal  vesicle  and  spot,  as  seen 
in  the  living  egg,  into  two  asters;  F,  formation  of  first  polar  cells  and  withdrawal 
of  remaining  part  of  nuclear  spindle  within  the  ovum;  G,  surface  view  of  living 
ovum  in  the  first  polar  cell;  H,  completion  of  second  polar  cell;  I,  a  later  stage, 
showing  the  remaining  internal  half  of  the  spindle  in  the  form  of  two  clear  vesi- 
cles; K,  ovum  with  two  polar  cells  and  radial  stria?  round  female  pronucleus,  as 
seen  in  the  living  egg  (E.  F,  H,  and  I  from  picric  acid  preparations);  L,  expulsion 
of  the  first  polar  cell.    (Haddon.) 

then,  are  simply  expelled  ;  they  take  no  part  in  the  development 
of  the  ovum  ;  and  their  extrusion  is  to  be  regarded  as  a  prepar- 
ation for  the  progress  of  the  cell,  whether  this  event  follows  or 
precedes  the  entrance  of  the  male  cell  into  the  ovum.  It  is  wor- 
thy of  note  that  the  ovum  may  become  amceboid  in  the  region 
from  which  the  polar  globules  are  expelled. 

The  remainder  of  the  nucleus( female  pronucleus)  now  passes 
inward  to  undergo  further  changes  of  undoubted  importance, 
possibly  those  by  virtue  of  which  all  the  subsequent  evolution 
of  the  ovum  is  determined.  This  brings  us  to  the  consideration 
of  another  cell  destined  to  play  a  brief  but  important  role  on  the 
biological  stage. 


REPRODUCTION. 


61 


THE    MALE    CELL    (SPERMATOZOON). 

This  cell,  almost  without  exception,  consists  of  a  nucleus 
(head)  and  vibratile  cilium.     However,  as  indicating  that  the 


Fig.  60.— Spermatozoa  (after  Haddon).  Not  drawn  to  scale.  1,  sponge;  2.  hydroid; 
3,  nematode:  4,  cray-fish;  5,  snail;  6,  electric  ray;  7,  salamander;  S.  horse;  9,  man. 
In  many  spermatozoa,  as  in  Nos.  7  and  9,  an  extremely  delicate  vibratile  band  is 
present. 

latter  is  not  essential,  spermatozoa  without  such  an  appendage 
do  occur.  The  obvious  purpose  of  the  cilium  is  to  convey  the 
male  cell  to  the  ovum  through  a  fluid  medium — either  the  water 
in  which  the  ova  are  discharged  in  the  case  of  most  invertebrates, 
or  through  the  fluids  that  overspread  the  surfaces  of  the  female 
generative  organs. 

The  Origin  of  the  Spermatozoon.—  The  structures  devoted  to 
the  production  of  male  cells  (testes),  when  reduced  to  their  es- 
sentials, consist  of  tubules,  of  great  length  in  mammals,  lined 


62 


COMPARATIVE   PHYSIOLOGY. 


with  nucleated  epithelial  cells,  from  which,  by  a  series  of 
changes  figured  above,  a  general  idea  of  their  development  may 
be  obtained. 

It  will  be  observed  that  throughout  the  series  the  nucleus  of 
the  cell  is  in  every  case  preserved,  and  finally  becomes  the  head 


fc'iG.  61.— Spermatogenesis.  A— H,  isolated  sperm-celis  of  the  rat,  showing  the  devel- 
opment of  the  spermatozoon  and  the  gradual  transformation  of  the  nucleus  into 
the  spermatozoon  head.  In  G  the  seminal  granule  is  being  cast  off  tatter  hi.  U. 
Brown)  I— M,  sperm-cells  of  an  Elasmobranch.  The  nucleus  ot  each  cell  divides 
into  a  large  number  of  daughter-nuclei,  each  one  of  which  is  converted  into  the 
rod-like  head  of  a  spermatozoon.  N,  transverse  section  of  a  ripe  cell,  showing 
the  bundle  of  spermatozoa  and  the  passive  nucleus  (I— N,  after  Semper).  O— b, 
spermatogenesis  in  the  earth-worm;  O,  young  sperm-cell;  P,  the  same  divided 
into  four;  (),  spermatosphere  with  the  central  sperm-blastophore;  R,  a  later  stage; 
S,  nearly  mature  spermatozoa.     (After  Blomfleld.) 


REPRODUCTION. 


63 


of  the  male  cell.     Once  more  we  are  led  to  see  the  importance 
of  this  structure  in  the  life  of  the  cell. 

Fertilization  of  the  Ovum. — The  spermatozoon,  lashing  its 
way  along,  when  it  meets  the  ovum,  enters  it  either  through  a 
special  minute  gateway  (micropyle),  or,  if  this  be  not  present— 
as  it  is  not  in  the  ova  of  all  animals — actually  penetrates  the 
membranes  and  substance  of  the  female  cell,  and  continues  act 
ive  till  the  female  pronucleus  is  reached,  when  the  head  enters 
and  the  tail  is  absorbed  or  blends  with  the  female  cell.  The  nu- 
cleus of  the  male  cell  prior  to  union  with  the  nucleus  of  the 


F.PNt 


F.PKt 


-M.PN. 


Fig.  6:2.— Fertilization  of  ovum  of  a  mollusk  (Elysia  viridis).  A.  Ovum  sending  up  a 
protuberance  to  meet  the  spermatozoon.  B.  Approach  of  male  pronucleus  to 
meet  the  female  pronucleus.  F.  FN,  female  pronucleus;  M.  FN,  male  pronucleus. 
S.  spermatozoon. 

ovum  undergoes  changes  similar  to  those  that  the  nucleus  of  the 
ovum  underwent,  and  thus  becomes  fitted  for  its  special  func- 
tions as  a  fertilizer  ;  or  perhaps  it  would  be  more  correct  to  say 
that  these  altered  masses  of  nuclear  substance  mutually  fertil- 
ize each  other,  or  initiate  changes  the  one  in  the  other  which 
conjointly  result  in  the  subsequent  stages  of  the  development 
of  the  ovum.  The  altered  male  nucleus  {male  pronucleus),  on 
reaching  the  female  pronucleus,  finds  it  somewhat  amaeboid, 
a  condition  which  may  be  shared  in  some  degree  by  the  entire 
ovum.  The  resulting  union  gives  rise  to  the  new  nucleus  {seg- 
mentation nucleus),  which  is  to  control  the  future  destinies  of 
the  cell  ;  while  the  cell  itself,  the  fertilized  ovum  {oosperm),  en- 
ters upon  new  and  marvelous  changes. 

In  reality  this  process  was  foreshadowed  in  the  dim  past  of 
the  history  of  living  things  by  the  conjugation  of  infusoria 
and  kindred  animal  and  vegetable  forms.  When  lower  forms 
(unicellular)  conjugate  they  become  somewhat  amoeboid  sooner 
or  later,  and  division  of  cell  contents  results.  In  some  cases 
Cseptic  monads)  the  resulting  cell  may  burst  and  give  rise  to  a 


64  COMPARATIVE   PHYSIOLOGY. 

shower  of  animal  dust  visible  only  by  the  highest  powers  of  the 
microscope,  each  particle  of  which  proves  to  be  the  nucleus 
from  which  a  future  individual  arises. 

The  study  of  reproduction  thus  establishes  the  conception  of 
a  unity  of  method  throughout  the  animal  and,  it  may  be  added, 
the  vegetable  kingdom,  for  reproduction  in  plants  is  in  all  main 
points  parallel  to  that  process  in  animals. 

But  why  that  costly  loss  of  protoplasm  by  polar  globules  ? 
For  the  present  we  shall  only  say  that  it  appears  necessary  to 
prevent  parthenogenesis  ;  or  at  least  to  balance  the  share  which 
the  male  and  female  elements  take  in  the  work  of  producing  a 
new  creature.  It  is  to  be  remembered  that  both  the  male  and 
female  lose  much  in  the  process — blood,  nervous  energy,  etc.,  in 
the  case  of  the  female,  while  the  male  furnishes  a  thousand-fold 
more  cells  than  are  used.  But  the  period  when  organisms  are 
best  fitted  for  reproduction  is  that  during  which  they  are  also 
most  vigorous,  and  can  best  afford  the  drain  on  their  super- 
fluous energies. 

SEGMENTATION   AND   SUBSEQUENT   CHANGES. 

After  the  changes  described  in  the  last  chapter  a  new  epoch 
in  the  biological  history  of  the  ovum — now  the  oosperm  (or  fer- 
tilized egg) — begins.  A  very  distinct  nucleus  (segmentation 
nucleus)  again  appears,  and  the  cell  assumes  a  circular  outline. 
The  segmentation  or  division  of  the  ovum  into  usually  fairly 
equal  parts  now  commences.  This  process  can  be  best  watched 
in  the  microscopic  transparent  ova  of  aquatic  animals  which 
undergo  perfect  development  up  to  a  certain  advanced  stage 
in  the  ordinary  water  of  the  ocean,  river,  lake,  etc.,  in  which 
the  adult  lives. 

Segmentation  among  invertebrates  will  be  first  studied,  and 
for  this  purpose  an  ovum  in  which  the  changes  are  of  a  direct 
and  uncomplicated  nature  will  be  chosen. 

The  following  figures  and  descriptions  apply  to  a  mollusk 
(Elysia  viridis).  We  distinguish  in  ova  resting  stages  and 
stages  of  activity.  It  is  not,  however,  to  be  supposed  that  abso- 
lute rest  ever  characterizes  any  living  form,  or  that  nothing  is 
transpiring  because  all  seems  quiet  in  these  little  biological 
worlds  ;  for  we  have  already  seen  reason  for  believing  that  life 
and  incessant  molecular  activity  are  inseparable.  It  may  be 
that,  in  the  case  of  resting  ova,  changes  of  a  more  active  char- 


REPRODUCTION. 


65 


acter  than  usual  are  going  on  in  their  molecular  constitution  ; 
hut,  on  the  other  hand,  there  may  be  really  a  diminution  of 


^I£j3____-£^?l 


Fig.  63.— Primitive  eggs  of  various  animals,  performing  amoeboid  movements  (.very 
much  enlarged).  "All  primitive  eggs  are  naked  cells,  capable  of  change  of  form. 
Within  the  dark,  finely  granulated  protoplasm  ^egg-yelk)  lies  a  large  vesicular 
kernel  (the  germ-vesicle'),  and  in  the  latter  is  a  nucleolus  (germ-spot);  in  the  nu- 
cleolus a  germ-point  (nucleolus)  is  often  visible.  Fig.  .-1  1 — ,-1  4.  The  primitive 
egg  of  a  chalk  sponge  {Leuculmis .echinus),  in  four  consecutive  conditions  of  mo- 
tion. Fia;.  B  1 — B  8.  The  primitive  egg  of  a  hermit-crab  (Ghondraeanthus  cornu- 
tux),  in  eight  consecutive  conditions  of  motion  (after  E.  Van  Beneden).  Fig.  C'l 
—  (75.  Primitive  egg  of  a  cat  in  four  different  conditions  of  motion  (after  Pfliiger). 
Fig.  D.  Primitive  egg  of  a  trout.  Fig.  E.  Primitive  egg  of  a  hen.  Fig.  F.  Primi- 
tive human  egg.    (Haeckel.) 

these  activities  in  correspondence  with  the  law  of  rhythm.    This 
seems  the  more  probable.    The  meaning,  however,  of  a  "  resting 


66 


COMPARATIVE  PHYSIOLOGY. 


stage  "  is  the  obvious  one  of  apparent  quiescence — cessation  of 
all  kinds  of  movement.  Then  ensues  rapidly  and  in  succession 
the  following  series  of  transformations  :  The  nucleolus  divides, 
later  the  nucleus,  into  two  parts.  These  new  nuclei  then  wan- 
der away  from  each  other  in  opposite  directions,  and,  losing 
their  character  as  nuclei  and  nucleoli,  are  replaced  by  asters 
(polar  stars),  which  seem  to  arise  in  the  protoplasm  of  the  body 


Fig.  64.— Early  stages  of  segmentation  of  a  mollusk,  Elysia  viridis  (drawn  from  the 
living  egg).  A,  oosperm  in  state  of  rest  after  the  extrusion  of  the  polar  cells;  B, 
the  nucleolus  alone  has  divided;  C,  the  nucleus  is  dividing;  D,  the  nucleus,  as 
such,  has  disappeared,  first  segmentation  furrow  appears;  E,  later  stage;  F, 
ofisperm  divided  into  two  distinct  segmentation  spheres,  the  clear  nuclear  space 
in  the  center  of  the  aster  of  granules  is  growing  larger;  G,  resting  stage  of  ap- 
pressed  two  spheres;  H,  I,  similar  stages  in  the  production  of  four  spheres;  K, . 
formation  of  eight-celled  stage.    (Haddon.) 


of  the  cell,  and  which  are  in  close  juxtaposition  at  first,  but  later 
separate,  the  oosperm  becoming  amoeboid  in  one  region  at  least. 
A  groove,  which  gradually  deepens,  appears  on  the  surface,  and 
finally  divides  the  cell  into  two  halves,  which  at  once  become 
flattened  against  each  other.  The  nucleus  may  again  be  recog- 
nized in  the  center  of  each  polar  star,  while  a  new  nucleolus 
also  reappears  within  the  nucleus,  when  again  a  brief  period  of 
rest  ensues.  In  the  division  and  reformation  of  the  nucleus, 
when  most  complicated  (karyoMnesis),  the  changes  may  be  gen- 


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REPRODUCTION. 


67 


eralizecl  as  consisting  of  division  and  segregation,  followed  by 
aggregation. 

The  subdivision  (segmentation)  of  the  cell,  after  the  quies- 
cence referred  to,  again  commences,  but  in  a  plane  at  right 
angles  to  the  first,  from  which  four  spheres  result,  again  to  be 
followed  by  the  resting  stage.  The  process  continues  in  the 
same  way,  so  that  there  is  a  progressive  increase  in  the  number 


Fig.  05.— The  cleavage  of  a  frog's  egg  (10  times  enlarged).  A,  the  parent-cell;  B,  the 
first  two  cleavage-cells;  0.4  cells;  D,  8  cells  (4  animal  and  4  vegetative);  E.  12 
cells  (8  animal  and  4  vegetative);  F,  16  cells  (8  animal  and  8  vegetative);  G,  24 
cells  (1(5  animal  and  8  vegetative);  H,  32  cells:  /.  48  cells;  A".  04  cell^:  L.  96  eleav- 
age-cclls;  .]/,  mo  cleavage-cells  (128  animal  and  32  vegetative).     (Haeckel.) 

of  segments,  at  least  up  to  the  point  when  a  large  number  has 
been  formed.     This  is  rather  to  be  considered  as  a  type  of  one 


68  COMPARATIVE   PHYSIOLOGY. 

form  of  segmentation  than  as  applicable  to  all,  for  even  at 
this  early  stage  differences  are  to  be  noted  in  the  mode  of  seg- 
mentation wbich  characterize  effectually  certain  groups  of  ani- 
mals ;  but  in  all  there  is  segmentation,  and  that  segmentation 
is  rhythmical. 

Segmentation  results  in  the  formation  of  a  multicellular 
aggregation  which,  sooner  or  later,  incloses  a  central  cavity 
(segmentation  cavity,  blastocele).  Usually  this  cell  aggrega- 
tion (blast  ula,  blastophere)  is  reduced  to  a  single  layer  of  invest- 
ing cells. 

The  Gastrula.— Ensuing  on  the  changes  just  described  are 

C 


Fig.  66.— .Blastula  and  gastrula  of  amphioxus  (Clans,  after  Hatschek).  A,  blastula 
with  flattened  lower  pole  of  larger  cells;  B,  commencing  invagination;  C,  gastru- 
lation  completed;  the  blastopore  is  still  widely  open,  and  one  of  the  two  hinder- 
pole  mesoderm  cells  is  seen  at  its  ventral  lip.  The  cilia  of  the  epiblast  cells  are 
not  represented. 

others,  which  result  in  the  formation  of  the  gastrula,  a  form  of 
cell  aggregation  of  great  interest  from  its  resemblance  to  the 
Hydra  and  similar  forms,  which  constitute  in  themselves  inde- 
pendent animals  that  never  pass  beyond  that  stage.  The  blas- 
tula becomes  flattened  at  one  pole,  then  depressed,  the  cells  at 
this  region  becoming  more  columnar  (histological  differentia- 
tion). This  depression  (invagination)  deepens  until  a  cavity  is 
formed  (as  when  a  hollow  rubber  ball  is  thrust  in  at  one  part 
till  it  meets  the  opposite  wall),  in  consequence  of  which  a  two- 
layered  embryo  results,  in  which  we  recognize  the  primitive 
mouth  (blastopore)  and  digestive  cavity  (archenterori),  the  outer 
layer  (ectoderm)  being  usually  separated  from  the  inner  (endo- 
derm)  by  the  almost  obliterated  segmentation  cavity.  Such  a 
form  may  be  provided  with  cilia,  be  very  actively  locomotive, 
and  bear,  consequently,  the  greatest  resemblance  to  the  perma- 
nent forms  of  some  aquatic  animals. 

The  changes  by  which  the  segmented  oosperm  becomes  a 
gastrula  are  not  always  so  direct  and  simple  as  in  the  above- 


REPRODUCTION. 


69 


described  case,  but  the' 
behavior  of  the  cells  of 
the  blastosphere  may  be 
hampered  by  a  burden 
of  relatively  foreign 
matter,  in  the  form  of 
food-yelk,  in  certain  in- 
stances ;  so  much  so  is 
this  the  case  that  dis- 
tinct modes  of  gastrula 
formation  may  be  rec- 
ognized as  dependent  on 
the  quantity  and  ar- 
rangement of  food-yelk. 
These  we  shall  pass  by 
as  being  somewhat  too 
complicated  for  our  pur- 
pose, and  we  return  to 
the  egg  of  the  bird. 

The  Hen's  Egg.— By 
far  the  larger  part  of 
the  hen's  egg  is  made 
up  of  yelk  ;  but  just 
beneath  the  vitelline 
membrane  a  small,  cir- 
cular, whitish  body, 
about  four  millimetres 
in  diameter,  which  al- 
ways floats  uppermost 
in  every  portion  of  the 
egg,  may  be  seen.  This 
disk  (blastoderm,  cica- 
tricula)  in  the  fertilized 
egg  presents  an  outer 
white  rim  (area  ojxica), 
within  which  is  a  trans- 
parent zone  (area  jielhi- 
cida),  and  most  centrally 
a  somewhat  elongated 
structure,  which  marks 
off  the  future  being 
itself      (embryo).       All 


Fig.  67.— Female  generative  organs  of  the  fowl 
(after  Dalton).  A,  ovary;  B,  Graafian  follicle, 
from  which  the  egg  has  just  been  discharged; 

C,  yelk,  entering  upon  extremity  of  oviduct: 

D,  E,  second  portion  of  oviduct,  "in  which  the 
chalaziferous  membrane,  chalazse.  and  albumen 
are  formed;  .F,  third  portion, in  which  the  fibrous 
shell  membranes  are  produced;  Q,  fourth  por- 
tion laid  open,  showing  the  egg  completely 
formed  with  its  calcareous  shell;  //,  canal 
through  which  the  egg  is  expelled. 


70 


COMPARATIVE   PHYSIOLOGY, 


these  parts  together  constitute  that  portion  (blastoderm)  of  the 
fowl's  egg  which  is  alone  directly  concerned  in  reproduction, 
all  the  rest  serving  for  nutrition  and  protection.  The  appear- 
ance of  relative  opacity  in  some  of  the  parts  marked  off  as  ahove 
is  to  be  explained  by  thickening  in  the  cell-layers  of  which  they 
are  composed. 

The  Origin  of  the  Fowl's  Egg.— The  ovary  of  a  young  but 
mature  hen  consists  of  a  mass  of  connective  tissue  (stroma), 


Fig.  08. — Various  stages  in  the  segmentation  of  a  fowl's 


Kolliker). 


abundantly  supplied  with  blood-vessels,  from  which  hang  the 
capsules  which  contain  the  ova  in  all  stages  of  development,  so 
that  the  whole  suggests,  but  for  the  color,  a  bunch  of  grapes  in 


REPRODUCTION.  71 

an  early  stage.  The  ovum  at  first,  in  this  case  as  in  all  others, 
a  single  cell,  becomes  complex  by  addition  of  other  cells  (dis- 
cus proligerus,  etc.),  which  go  to  make  up  the  yelk.  All  the 
other  parts  of  the  hen's  egg  are  additions  made  to  it,  as  ex- 
plained before,  in  its  passage  down  the  oviduct.  The  original 
ovum  remains  as  the  blastoderm,  the  segmentation  of  which 
may  now  be  described  briefly,  its  character  being  obvious  from 
an  examination  of  Fig.  68,  which  represents  a  surface  view  of 
the  segmenting  fertilized  ovum  (oosperm). 

A  segmentation  cavity  appears  early,  and  is  bounded  above 
by  a  single  layer  of  epiblast  cells  and  below  by  a  single  layer  of 
primitive  hypoblast  cells,  which  latter  is  soon  composed  of  sev- 
eral layers,  while  the  segmentation  cavity  disappears. 

The  blastoderm  of  an  unincubated  but  fertilized  egg  consists 
of  a  layer  of  epiblastic  cells,  and  beneath  this  a  mass  of  rounded 
cells,  arranged  irregularly  arid  lying  loosely  in  the  yelk,  consti- 
tuting the  primitive  hypoblast.  After  incubation  for  a  couple 
of  hours,  these  cells  become  differentiated  into  a  lower  layer  of 
flattened  cells  (hypoblast),  with  mesoblastic  cells  scattered  be- 


Fig.  69.— Portion  of  section  through  an  unincubated  fowl's  oOsperm  (after  Klein). 
«,  epiblast  composed  of  a  single  layer  of  columnar  cells;  b,  irregularly  disposed 
lower  layer  cells  of  the  primitive  hypoblast;  c,  larger  formative  cells  resting  on 
white  yelk;  f,  archenteron.  The  segmentation  cavity  lies  between  a  and  b,  and 
is  nearly  obliterated. 

tween  the  epiblast  and  hypoblast,  It  is  noteworthy  that,  in  the 
bird,  segmentation  will  proceed  up  to  a  certain  stage  independ- 
ently of  the  advent  of  the  male  cell,  apparently  indicating  a 
tendency  to  parthenogenesis. 

The  fowl's  ovum  then  belongs  to  the  class,  a  portion  of  which 
alone  segments  and  develops  into  the  embryo  (meroblastic),  in 
contradistinction  to  what  happens  in  the  mammalian  ovum,  the 
whole  of  which  undergoes  division  (holoblastic)  ;  a  distinction 
which  is,  however,  superficial  rather  than  fundamental,  for  in 
reality  in  the  fowl's  egg  the  whole  of  the  original  pvum  does 
segment.     This  holoblastic  character  of  the  mammalian  ovum 


72 


COMPARATIVE  PHYSIOLOGY. 


and  its  resemblance  to  the  segmentation  of  those  invertebrate 
forms  previously  described  may  become  apparent  from  an  ex- 
amination of  the  accompanying  figures. 


Fig.  70.— Sections  of  ovum  of  a  rabbit,  illustrating  formation  of  the  blastodermic  vesi- 
cle (after  E.  Van  Beneden).  A,  B,  C,  D,  are  ova  in  successive  stages  of  develop- 
ment, zp,  zonapellucida;  eel,  ectomeres,  or  outer  cells;  ent,  entomeres,  or  inner 
cells. 

We  shall  return  to  the  development  of  the  mammalian  ovum 
later ;  in  the  mean  time  we  present  the  main  features  of  develop- 
ment in  the  bird. 

Remembering  that  the  development  of  the  embryo  proper 
takes  place  within  the  pellucid  area  only,  we  point  out  that  the 
area  opaca  gradually  extends  over  the  entire  ovum,  inclosing  the 
yelk,  so  that  the  original  disk  which  lay  like  a  watch-glass  on 
the  rest  of  the  ovum,  has  grown  into  a  sphere.  That  portion 
of  this  area  nearest  the  pellucid  zone  {area  vasculosa)  develops 
blood-vessels  that  derive  the  food-supplies,  which  replenish  the 
blood  as  it  is  exhausted,  from  the  hypoblast  of  the  area  opaca. 


REPRODUCTION. 


73 


The  first  indications  of  future  structural  outlines  in  the  em- 
bryo is  the  formation  of  the  primitive  streak,  an  opaque  band 


Pig.  71.— Diagrammatic  transverse  sections  through  a  hypothetical  mammal  oOsperm 
(Haddon).  A.  The  yelk  of  the  primitive  mammalian  oOspenn  is  now  lost.  B. 
Later  stage;  the  non-embryonic  epiblast  has  grown  over  the  embryonic  area  to 
form  the  covering  cells;  ep.  epiblast  of  embryo;  ep'.  epiblast  of  yelk-sac;  7uj, 
primitive  hypoblast;  y.  s,  yelk-sac,  or  blastodermic  vesicle. 

in  the  long  diameter  of  the  pellucid  area,  opaque  in  consequence 
of  cell  accumulation  in  that  region.  Very  soon  a  groove  {primi- 
tive groove)  extends  through- 
out this  band,  which  gradu- 
ally occupies  a  more  central 
position.  The  relative  thick- 
ness of  the  several  parts  and 
the  arrangement  of  cells  may 
be  gathered  from  Fig  72. 
These  structures  are  only 
temporary,  and  those  that  re- 
place them  will  be  described 
subsequently. 

We  have  thus  far  spoken 
of  cells  as  being  arranged  in- 
to epiblast,  hypoblast,  and 
mesoblast.  The  origin  of  the 
first  two  has  been  sufficiently 
indicated.  The  mesoblast 
forms  the  intermediate  ger- 
minal layer,  and  is  derived 
from  the  primitive  hypoblast, 
which  differentiates  into  a 
stratum  of  flattened  cells, 
situated  below  the  others, 
and   constituting    the    later 


i 


Fig.  T2.-Surface  view  of  pellucid  area  of 
blastoderm  of  eighteen  hours  (Foster  and 
Balfour).  //.  medullary  folds  ;  me,  me- 
dullary groove;  pr,  primitive  groove. 


u 


COMPARATIVE  PHYSIOLOGY. 


hypoblast,  and  intermediate  less  closely  arranged  cells,  termed, 
from  their  position,  mesoblast. 

It  will  be  noticed  that  all  future  growth  of  the  embryo  be- 
gins axially,  at  least  in  the  early  stages  of  its  development. 

As  the  subsequent  growth  and  advance  of  the  embryo  de- 
pend on  an  abundant  and  suitable  nutritive  supply,  we  must 
now  turn  to  those  arrangements  which  are  temporary  and  of 
subordinate  importance,  but  still  for  the  time  essential  to  devel- 
opment. 

THE   EMBRYONIC    MEMBRANES   OF    BIRDS. 

It  will  be  borne  in  mind  throughout  that  the  chief  food-sup- 
ply for  the  embryo  bird  is  derived  from  the  yelk ;  and,  as  would 


—pp 


■vf. 


Vt 


w 


Pig.  73. 

Figs.  73-?5.— A  series  of  diagrams  intended  to  facilitate  the  comprehension  of  the 
relations  of  the  membranes  to  other  parts  (after  Foster  and  Balfour).  A,  B,  C,  D, 
E,  F  are  vertical  sections  in  the  long  axis  of  the  embryo  at  different  periods  show- 
ing the  stages  of  development  of  the  amnion  and  of  the  yelk-sac.  I,  II,  III,  IV 
are  transverse  sections  at  about  the  same  stages  of  development,  i,  ii,  iii,  pos- 
terior part  of  longitudinal  section,  to  illustrate  three  stages  in  formation  of  the 
allantois.  e,  embryo;  y,  yelk;  pp,  pleuroperitoneal  cavity;  vt,  vitelline  mem- 
brane of  amniotic  fold;'«/,  allantois;  a,  amnion;  a',  alimentary  canal. 

be  expected,  the  older  the  embryo  the  smaller  the  yelk,  or,  as  it 
is  now  called  when  limited  by  the  embryonic  membranes,  the 


REPRODUCTION, 


75 


\\    m 


n 


76 


COMPARATIVE  PHYSIOLOGY. 


yelk-sac  {umbilical  vesicle  of  the  mammalian  embryo).  The 
manner  in  which  this  takes  place  will  appear  npon  an  inspec- 
tion of  the  accompanying  figures. 

Very  early  in  the  history  of  the  embryo  two  eminences,  the 
head  and  the  tail  folds,  arise,  and,  curving  over  toward  each 


MO. 


Fig.  70. — Diagrammatic  longitudinal  section  through  the  axis  of  an  embryo  chick 
(after  Foster  and  Balfour).  N.  C,  Neural  canal;  Ch,  notochord;  Fg,  foregnt; 
F.  So,  somatopleure;  F.  Sp,  splanchnopleure;  Sp,  splanchnopleure,  forming  lower 
wall  of  foregut;  lit,  heart;  pp,  pleuroperitoneal  cavity;  Am,  amniotic  fold;  E, 
epiblast;  M,  mesoblast;  H,  hypoblast. 

other,  meet  after  being  joined  by  corresponding  lateral  folds. 
Fusion  and  absorption  result  at  this  meeting-point,  in  the 
inclosure  of  one  cavity  and  the  blending  of  two  others.  These 
folds  constitute  the  amniotic  membranes,  the  inner  of  which 


Fig.  77.— Diagrammatic  longitudinal  section  of  a  chick  of  the  fourth  day  (after  Allen 
Thomson),  ep,  epiblast;  hy,  hypoblast;  sm,  somatopleure;  vm,  splanchnopleure; 
of,  pf,  folds  of  the  amnion;  pp,  pleuroperitoneal  cavity;  am,  cavity  of  the  am- 
nion'; at.  allantois;  a,  position  of  the  future  anus;  h,  heart;  i,  intestine;  vi,  vitel- 
line duct;  ye,  yelk;  x,  foregut;  m,  position  of  the  mouth;  me,  mesentery. 

forms  the  true  amnion,  the  outer  the  false  amnion  (serous  mem- 
brane, subzonal  membrane).  Within  the  amnion  proper  is  the 
amniotic  cavity  filled  with  fluid  (liquor  amnii),  while  the  space 
between  the  true  and  false  amniotic  folds,  which  gradually  in- 


REPRODUCTION. 


77 


creases  in  size  as  the  yelk-sac  diminishes,  forms  the  pleuro- 
peritoneal  cavity,  body  cavity,  or  coclom.  The  amniotic  cavity 
also  extends,  so  that  the  embryo  is  surrounded  by  it  or  lies 
centrally  within  it.  The  enlargement  of  the  ccelom  and  exten- 
sion of  the  false  amniotic  folds  lead  finally  to  a  similar  meeting 
and  fusion  like  that  which  occurred  in  the  formation  of  the  true 
amniotic  cavity.  The  yelk-sac,  gradually  lessening,  is  at  last 
withdrawn  into  the  body  of  the  embryo. 

Fig.  76  shows  how  the  amniotic  head  fold  arises,  from  a 
budding  out  of  the  epiblast  and  mesb  blast  at  a  point  where  the 
original  cell  layers  of  the  embryo  have  separated  into  two  folds, 
the  somatopleure  or  body  fold  and  the  splanchnoplenre  or  vis- 
ceral fold,  owing  to  a  division  or  cleavage  of  the  mesoblast 
toward  the  long  axis  of  the  body.  Remembering  this,  it  is 
always  easy  to  determine  by  a  diagram  the  composition  of  any 
one  of  the  membranes  or 
folds  of  the  embryo,  for 
the  components  must  be 
epiblast,  mesoblast,  or 
hypoblast  ;  thus,  the 
splanclmopleure  is  made 
up  of  hypoblast  internally 
and  mesoblast  externally 
— a  principle  of  great  sig- 
nificance, since,  as  will  be 
learned  later,  all  the  tis- 
sues of  the  body  may  be 
classified  simply,  and  at 
the  same  time  scientifi- 
cally, according  to  their 
embryological  origin. 

The  allantois  is  a 
structure  of  much  physi- 
ological importance.  It 
arises  at  the  same  time  as 
the    amniotic    folds    are 

forming,  by  a  budding  or  protrusion  of  the  hind-gut  into  the 
pleuro-peritoueal  cavity,  and  hence  consists  of  an  outgrowth  of 
mesoblast  lined  by  hypoblast. 


Fig.  78.  —  Diagrammatic  longitudinal  section 
through  the  egg  of  a  fowl  (after  Duval). 
al,  cavity  of  allantois;  alb.  albumen;  a/'t.  mes- 
enteron;"  am,  cavity  of  amnion;  emb,  embryo; 
sh.  egg-shell;  v.  m,  vitelline  membrane. 


COMPARATIVE   PHYSIOLOGY. 


THE   FCETAL   (EMBRYONIC)   MEMBRANES   OF 
MAMMALS. 

The  differences  between  the  development  of  the  egg  mem- 
branes of  mammals  and  birds  are  chiefly  such  as  result  from 

the  absence  in  the 
former  of  an  egg-shell 
and  its  membranes,  and 
of  yelk  and  albumen. 
The  mammalian  ovum 
is  inclosed  by  a  zona 
radiata  {zona  pelluci- 
da)  surrounded  by  an- 
other very  delicate  cov- 
ering (vitelline  mem- 
brane). 

The  growth  of  the 
blastodermic  vesicle 
(yelk-sac)  is  rapid,  and, 
being  filled  with  fluid, 
the  zona  is  thinned  and 
soon  disappears. 

The  germinal  area 
alone  is  made  up  of 
three  layers  of  cells 
(Fig.  100),  the  rest  of 
the  upper  part  of  the 
oosperm  being  lined 
with  epiblast  and  hyp- 
oblast, while  the  low- 
er zone  of  the  yelk-sac  consists  of  epiblast  only. 

Simple,  non-vascular  villi,  serving  to  attach  the  embryo 
to  the  uterine  walls,  usually  project  from  the  epiblast  of  the 
subzonal  membrane.  In  the  rabbit  they  do  not  occur  every- 
where, but  only  in  that  region  of  the  epiblast  beneath  which 
the  mesoblast  does  not  extend,  with  the  exception  of  a  patch 
which  soon  appears  and  demarkates  the  site  of  the  future  pla- 
centa. The  amnion  and  allantois  are  formed  in  much  the  same 
way  as  has  been  described  for  the  chick. 

At  about  the  same  period  as  these  events  are  transpiring  the 
vascular  yelk-sac  has  become  smaller,  and  the  allantois  with 
its  abundant  supply  of  blood-vessels  is  becoming  more  promi- 


Fig.  79. — Diagrammatic  longitudinal  section 
oosperm  of  rabbit  at  an  advanced  stage  of  preg- 
nancy (KOlliker,  after  Bischoff).  a,  amnion;  al, 
allantois  with  its  blood-vessels;  e,  embryo  ;  ds, 
yelk-sac  ;  ed,  ed' ,  ed",  hypoblastic  epithelium  of 
the  yelk-sac  and  its  stalk  (umbilical  vesicle  and 
cord);  fd,  vascular  mesoblastic  membrane  of  the 
umbilical  cord  and  vesicle  ;  pi,  placental  villi 
formed  by  the  allantois  and  subzonal  membrane; 
r,  space  filled  with  fluid  between  the  amnion, 
the  allantois.  and  the  yelk-sac  ;  ft,  sinus  termi- 
nalis  (marginal  vitelline  blood-vessel);  u,  urach- 
us,  or  stalk  of  the  allantois. 


REPRODUCTION. 


79 


^4-^ro'/n* 


nent,  and  extending  between  the  amnion  and  subzonal  mem- 
brane. 

The  formation  of  the  chorion  marks  an  important  step  in 
the  development  of  mammals  in  which  it  plays  an  important 
functional  part.  It  is 
the  result  of  the  fusion 
of  the  allantois,  which 
is  highly  vascular, 
with  the  subzonal 
membrane,  the  villi  of 
which  now  become 
themselves  vascular 
and  more  complex  in 
other  respects. 

An  interesting  re- 
semblance to  birds  has 
been  observed  (by  Os- 
born)  in  the  opossum 
(Fig.  83).  When  the 
allantois  is  small  the 
blastodermic  vesicle 
(yelk-sac)  has  vascular 
villi,  which  in  all  prob- 
ability not  only  serve 
the  purpose  of  attach- 
ing the  embryo  to  the 
uterine  wall  but  derive 
nourishment,  not  as  in 

birds,  from  the  albumen  of  the  ovum,  but  directly  in  some  way 
from  the  uterine  wall  of  the  mother.  It  will  be  remembered 
that  the  opossum  ranks  low  in  the  mammalian  scale,  so  that  this 
resemblance  is  the  more  significant  from  an  evolutionary  point 
of  view. 

The  term  chorion  is  now  restricted  to  those  regions  of  the 
subzonal  membrane  to  which  either  the  yelk-sac  or  the  allan- 
tois is  attached.  The  former  zone  has  been  distinguished  as  the 
false  chorion  and  the  latter  as  the  true  chorion.  In  the  rabbit 
the  false  chorion  is  very  large  (Fig.  79),  and  the  true  (placen- 
tal) chorion  very  small  in  comparison,  but  the  reverse  is  the 
case  in  most  mammals.  It  will  be  noted  that  in  both  birds 
and  mammals  the  allantois  is  a  nutritive  organ.  Usually 
the  more  prominent  and   persistent  the  yelk-sac,  the  less  so 


Fig.  80.— Diagrammatic  dorsal  view  of  an  embryo  rab- 
bit with  its  membranes  at  the  stage  of  nine  so- 
mites (Hadclon,  after  Van  Beneden  and  Julin). 
aly  allantois,  showing  from  behind  the  tail  fold 
of  the  embryo;  am,  anterior  border  of  true  am- 
nion ;  a.  v,  area  vasculosa,  the  outer  border  of 
which  indicates  the  farthest  extension  of  the 
mesoblast;  hi,  blastoderm,  here  consisting  only  of 
epiblast  and  hypoblast;  o.  m.  v,  omphalomesen- 
teric or  vitelline  veins  ;  p.  am,  proamnion  ;  pi, 
non-vascular  epiblastic  villi  of  the  future  placen- 
ta ;  s.  t,  sinus  terminalis. 


80 


COMPARATIVE   PHYSIOLOGY. 


the  allantois,  and  vice  versa ;  they  are  plainly  supplementary 
organs. 

The  Allantoic  Cavity.— The  degree  to  which  the  various  em- 
bryonic membranes  fuse  together  is  very  variable  for  different 
groups  of  mammals,  including  our  domestic  species. 

In  ruminants,  but  especially  in  solipeds,  the  allantois  as  it 
grows  spreads  itself  over  the  inner  surface  of   the  subzonal 


'Mm- 


Fig.  81.— Embryo  of  dog.  twenty-five  days  old.  opened  on  the  ventral  side.  Chest 
and  ventral  walls  have  been  removed,  a,  nose-pits;  b,  eyes;  c,  nnder-jaw  (first 
gill-arch);  d,  second  gill-arch;  e,f,  0,  h,  heart  (e,  right,/,  left  auricle;  g,  right,  h, 
left  ventricle);  i,  aorta  (origin  of);  kk,  liver  (in  the  middle  between  the  two  lobes 
is  the  cut  yelk-vein);  /,  stomach;  m,  intestine;  n,  yelk-sac;  o,  primitive  kidneys; 
p,  allantois;  q,  fore-limbs;  h,  hind-limbs.  The  crooked  embryo  has  been  stretched 
straight.    (Ilaeckel,  after  Bischoff.) 

membrane,  often  spoken  of  as  the  "  chorion,"  while  it  also 
covers,  though  capable  of  easy  detachment,  the  outer  surface  of 
the  amnion ;  and  thus  is  formed  the  allantoic  cavity.  The  por- 
tion of  the  allantois  remaining  finally  within  the  foetus  becomes 
the  bladder,  which  during  embryonic  life  communicates  by  its 
contracted  portion  (urachus)  with  the  general  amniotic  cavity. 


REPRODUCTION. 


81 


Fig.  82. — Diagram  of  an  embryo  showing  the  relations  of  the  vascular  allantois  to  the 
villi  of  the  chorion  (Cadiat).  e,  embryo  lying  in  the  cavity  of  the  amnion;  y$, 
yelk-sac;  al,  allantois;  A.Y,  allantoic  vessels  dipping  into  the  villi  of  the  chorion; 
ch.  chorion. 


-\-aro. 


In  the  mare  especially  these  parts  can  be  readily  distin- 
guished. From  the  connection  of  the  portion  that  ultimately 
forms  the  bladder  with  the 
main  sac,  as  indicated 
above,  there  is  ground  for 
regarding  the  allantoic 
fluid  in  the  later  stages  of 
gestation,  at  all  events,  as 
a  sort  of  urine. 

This  fluid  is  at  an  ear- 
ly  period  abundant  and 
colorless,  later  yellowish, 
and  finally  brown.  Since 
at  one  time  it  contains 
albumen  and  sugar,  it 
may  serve  some  purpose 
in  the  nutrition  of  the 
foetus. 

When  most  suggestive 
of  urine  in  the  latest  stages 
of    gestation,    it    contains 
6 


Fig.  83.— Diagram  of  the  foetal  membranes  of 
the  Virginian  opossum  (Haddon,  after  Os- 
born).  Two  villi  are  shown  greatly  enlarged. 
The  processes  of  the  cells,  which  have  been 
exaggerated,  doubtless  correspond  to  the 
pseudopodia  described  by  Caldwell,  al, 
allantois;  am,  amnion:  s.t,  sinus  termi- 
nalis;  <«. z,  subzonal  membrane;  r.  villi  on 
the  subzonal  membrane  in  the  region  of 
the  yelk-sac ;  ijs.  yelk-sac.  The  vascular 
splanchuopleure  (hypoblast  and  mesoblast) 
is  indicated  by  the  "black  line. 


82 


COMPARATIVE  PHYSIOLOGY. 


a  characteristic  body,  allantoin,  related  to  uric  acid,  urea, 
etc. 

Certain  bodies,  being  probably  inspissated  allantoic  fluid, 
have  been  termed  "  hippomanes. "  They  may  either  float  free 
in  the  fluid  or  be  attached  to  the  allantois  by  a  slender  pedicle. 

The  relation  of  the  parts  described  above  will  become  clearer 
after  a  study  of  the  accompanying  cuts  and  those  of  preceding 
pages,  in  which  the  allantois  is  figured. 


Fig.  84.— Exterior  of  chorial  sac;  mare.    (Chauveau.)    A,  body;  B.  C.  cornua. 

The  Placenta. — This  structure,  which  varies  greatly  in  com- 
plexity, may  be  regarded  as  the  result  of  the  union  of  structures 
existing  for  a  longer  or  shorter  period,  free  and  largely  inde- 
pendent of  each  other.  With  evolution  there  is  differentiation 
and  complication,  so  that  the  placenta  usually  marks  the  site 
where  structures  have  met  and  fused,  differentiating  a  new  or- 
gan; while  corresponding  atrophy,  obliteration,  and  fusion  take 
place  in  other  regions. 

All  placentas  are  highly  vascular,  all  are  villous,  all  dis- 
charge similar  functions  in  providing  the  embryo  with  nourish- 
ment and  eliminating  the  waste  of  its  cell-life  (metabolism). 
In  structural  details  they  are  so  different  that  classifications  of 
mammals  have  been  founded  upon  their  resemblances  and  dif- 
ferences.    They  will  now  be  briefly  described. 

In  marsupials  the  yelk-sac  is  both  large  and  vascular;  the 
allantois  small  but  vascular;  the  former  is  said  (Owen)  to  be 
attached  to  the  subzonal  membrane,  the  latter  not;  but  no  villi, 
and  consequently  no  true  chorion,  is  developed.     All  mammals 


REPRODUCTION. 


83 


Pig.  85. — Foetus  of  mare  with  its  envelopes.  (Chauveau.)  A.  chorion:  C,  amnion  re- 
moved from  allantoicl  cavity  and  opened  to  expose  foetus;  D,  infundibulum  of 
urachus;  B,  allantoid  portion  of  umbilical  cord. 

other  than  the  monotremes  and  marsupials  have  a  true  allan- 
toic placenta. 

The  Discoidal  Placenta. — This  form  of  placenta  is  that  exist- 
ing in  the  rodentia,  insectivora,  and  cheiroptera.  The  condition 
found  in  the  rabhit  is  that  which  has  been  most  studied.  The 
relation  of  parts  is  shown  in  Fig;.  79. 

The  uterus  of  the  rodent  is  two-homed ;  so  we  find  in  gen- 
eral several  embryos  in  each  horn  in  the  pregnant  rabbit. 
They  are  functionally  independent,  each  having  its  own  set  of 


84 


COMPARATIVE  PHYSIOLOGY. 


membranes.  It  will  be  observed  from  the  figure  tbat  tbe  true 
villous  cborion  is  confined  to  a  comparatively  small  region; 
tbere  is,  however,  in  addition  a  false  cborion  witbout  villi,  but 
bigbly  vascular.  This  blending  of  forms  of  placentation  wbicb 
exist  separately  in  different  groups  of  animals  is  significant. 
In  tbe  rabbit  at  a  later  stage  tbere  is  considerable  interming- 
ling of  foetal  and  maternal  parts,, 


i  %~ Series  of  diagrams  representing  the  relations  of  the  decidna  to  the  ovum,  at 
different  periods,  in  the  human  subject.  The  decidua  are  dark,  the  ovum  shaded 
transversely.  In  4  and  5  the  chorionic  vascular  processes  are  figured  (after  Dal- 
ton),  1.  Ovum  resting  on  the  decidua  serotina;  2.  Decidua  reflexa  growing  round 
th'- ovum;  3.  Completion  of  the  decidua  around  the  ovum;  4.  Villi,  growing  out 
all  around  the  chorion;  5.  The  villi,  specially  developed  at  the  site  of  the  future 
placenta,  having  atrophied  elsewhere. 

The  Metadiscoidal  Placenta.— This  type,  which,  in  general 
naked-eye  appearances,  greatly  resembles  the  former,  is  found 
in  man  and  the  apes.  The  condition  of  things  in  man  is  by  no 
means  as  well  understood  as  in  the  lower  mammals,  especially 
in  the  early  stages ;  so  that,  while  the  following  account  is  that 


REPRODUCTION. 


85 


usually  given  in  works  on  embryology,  the  student  may  as  well 
understand  that  our  knowledge  of  human  embryology  in  the 
very  earliest  stages  is  incomplete  and  partly  conjectural.  The 
reason  of  this  is  obvious :  specimens  for  examination  depending 
on  accidents  giving  rise  to  abortion  or  sudden  death,  often  not 
reaching  the  laboratory  in  a  condition  permitting  of  trust- 
worthy inferences. 

It  is  definitely  known  that  the.  ovum,  which  is  usually  fer- 
tilized in  the  oviduct  (Fallopian  tube),  on  entering  the  uterus 
becomes  adherent  to  its  wall  and  encapsuled.  The  mucous 
membrane  of  the  uterus  is  known  to  undergo  changes,  its  com- 
ponent parts  increasing  by  cell  multiplication,  becoming  in- 
tensely vascular  and  functionally  more  active.  The  general 
mucous  surface  shares  in  this,  and  is  termed  the  decidua  vera  ; 
but  the  locality  where  the  ovum  lodges  is  the  seat  of  the  great- 
est manifestation  of  exalted  activity,  and  is  termed  the  decidua 
serotina  ;  while  the  part  believed  to  have  invested  the  ovum  by 


Fig.  87.— Vascular  system  of  the  hnman  frettis,  represented  diagrammatical]}-  (TTnx- 
ley).  77.  heart:  TA,  aortic  trunk:  c,  common  carotid  artery:  r'.  external  carotid 
artery:  c".  internal  carotid  artery:  a,  subclavian  artery:  v,  vertebral  artery:  1.  '.?. 
3.  4.  5.  aortic  arches:  A',  dorsal  aorta:  o.  omphalo-mesenterie  artery;  <h\  vitelline 
duct:  r>'.  omphalo-mesenterie  vein;  v1,  umbilical  vesicle;  vp,  portal  vein:  L.  liver; 
a.  v.  umbilical  arteries:  u",  v".  their  endincrs  in  the  placenta:  '/'.  umbilical  vein; 
Dr.  ductus  venosus;  rh.  hepatic  vein:  rr.  inferior  vena  cava:  vit,  iliac  veins;  az, 
vena  azygos;  vc',  posterior  cardinal  vein;  DC,  duct  of  Cuvier;  P.  lung. 


86 


COMPARATIVE   PHYSIOLOGY. 


fused  growths  from,  the  junction  of  the  decidua  vera  and  sero- 
tina  is  the  decidua  reflexa. 

The  decidua  serotina  and  reflexa  thus  become  the  outermost 
of  all  the  coverings  of  the  ovum.  These  and  some  other  devel- 
opments are  figured  below.  It  is  to  be  remembered,  however, 
that  they  are  highly  diagrammatic,  and  represent  a  mixture  of 
inferences  based,  some  of  them,  on  actual  observation  and  others 
on  analogy,  etc. 

The  figures  will  convey  some  information,  though  appear- 
ances in  all  such  cases  must  be  interpreted  cautiously  for  the 
reasons  already  mentioned. 

During  the  first  fourteen  days  villi  appear  over  the  whole 
surface  of  the  ovum  ;  about  this  fact  there  is  no  doubt.  At 
the  end  of  the  first  month  of  fcetal  life,  a  complete  chorion 
has  been  formed,  owing,  it  would  seem,  to  the  growth  of  the 
allantois  (its  mesoblast  only)  beneath  the  whole  surface  of  the 
subzonal  membrane.  From  the  chorionic  surface  vascular  pro- 
cesses clothed  with  epithelium  project  like  the  plush  of  velvet. 
The  allantois  is  compressed  and  devoid  of  a  cavity,  but  abun- 
dantly supplied  with  blood-vessels  by  the  allantoic  arteries  and 
veins,  which  of  course  terminate  in  capillaries  in  the  villi. 
Compare  the  whole  series  of  figures. 


Fig.  88.—  Human  ova  during  early  stages  of  development.  A  and  B,  front  and  side 
view  of  an  ovum  supposed  to  be  about  thirteen  days  old;  e.  embryonic  area 
(Quain,  after  Reichert);  C,  ovrnn  of  four  to  five  weeks,  showing  the  general 
structure  of  the  ovum  before  formation  of  the  placenta.  Partof  the  wall  or  me 
ovum  is  removed  to  show  the  embryo  in  position  (after  Allen  Thomson). 

At  this  stage  the  condition  of  the  chorion  suggests  the  type 
of  the  diffuse  placenta  which  is  normal  for  certain  groups  of 
animals,  as  will  presently  be  learned. 

The  subsequent  changes  are  much  better  understood,  for 
parts  are  in  general  no  longer  microscopic  but  of  considerable 
size,  and  their  real  structure  less  readily  obscured  or  obliterated. 

The  amniotic  cavity  continues  to  enlarge  by  growth  of  the 
walls  of  the  amnion  and  is  kept  filled  with  a  fluid;  the  yelk-sac 


REPRODUCTION. 


87 


is  now  very  small  ;  the  decidua  renexa  becomes  almost  non- 
vascular, and  fuses  finally  with  the  decidua  vera  and  the  cho- 
rion, which  except  at  one  part  has  ceased  to  be  villous  and  vas- 
cular ;  so  that  becoming  thinner  and  thinner  with  the  advance 
of  pregnancy,  the  single  membrane,  arising  practically  from 

I  d 


Fig.  89.— Human  embryo,  twelve  weeks  old,  with  its  coverings;  natural  size.  The 
navel-cord  passes  from  the  navel  to  the  placenta,  b,  amnion;  c,  chorion;  d,  pla- 
centa; d',  remains  of  tufts  on  the  smooth  chorion;  /.  decidua  reflexa  (inner);  g, 
decidua  vera  (outer).    (Haeckel  after  Bernhard  Schu'ltze.) 

this  fusion  of  several,  is  of  a  low  type  of  structure,  the  result  of 
gradual  degeneration,  as  the  role  they  once  played  was  taken 
up  by  the  other  parts. 

But  of  paramount  importance  is  the  formation  of  the  pla- 
centa. The  chorion  ceases  to  be  vascular  except  at  the  spot  at 
which  the  villi  not  only  remain,  but  become  more  vascular  and 
branch  into  arborescent  forms  of  considerable  complexity.  It 
is  discoidal  in  form,  made  up  of  a  fcetal  part  just  described  and 
a  maternal  part,  the  decidua  serotina,  the  two  becoming  blended 


COMPARATIVE   PHYSIOLOGY. 


so  that  the  removal  of  one  involves  that  of  more  or  less  of  the 
others.     The  connection  of  parts  is  far  closer  than  that  described 


Fig.  90.— Diagram  illustrating  the  decidua,  placenta,  etc.  (after  Liegeois).  e,  embryo; 
i,  intestine:  p,  pedicle  of  the  umbilical  vesicle;  u.  v,  umbilical  vesicle;  a,  amnion; 
eh,  chorion;  v.  t,  vascular  tufts  of  the  chorion,  constituting  the  fcetal  portion  of 
the  placenta;  m.p,  maternal  portion  of  the  placenta;  d.  v,  decidua  vera;  d.  r,  de- 
cidua reflexa;  al,  allantois. 

for  the  rabbit ;  and,  even  with  the  preparation  that  Nature 
makes  for  the  final  separation  of  the  placenta  from  both  foetus 
and  mother,  this  event  does  not  take  place  without  some  rupture 
of  vessels  and  consequent  haemorrhage. 

It  is  difficult  to  conceive  of  the  great  vascularity  of  the 
human  placenta  without  an  actual  examination  of  this  structure 
itself,  which  can  be  done  after  being  cast  off  to  great  advan- 
tage when  floating  in  water ;  by  which  simple  method  also  the 
thinness  and  other  characteristics  of  the  membranes  can  be 
well  made  out. 

The  great  vessels  conveying  the  fcetal  blood  to  and  from  the 


REPRODUCTION.  89 

placenta  are  reduced  to  three,  two  arteries  and  one  vein.  The 
villi  of  the  placenta  (chorion)  are  usually  said  to  hang  freely 
in  the  blood  of  the  large  irregular  sinuses  of  the  decidua  sero- 
tina;  but  this  is  so  unlike  what  prevails  in  other  groups  of 
animals  that  we  can  not  refrain  from  believing  that  the  state- 
ment is  not  wholly  true. 

The  Zonary  Placenta. — In  this  type  the  placenta  is  formed 
along  a  broad  equatorial  belt,  leaving  the  poles  free.  This  form 
of  placentation  is  exemplified  in  the  carnivora,  hyrax,  the  ele- 
phant, etc. 

In  the  dog,  for  example,  the  yelk-sac  is  large,  vascular,  does 
not  fuse  with  the  chorion,  and  persists  throughout.  A  rudi- 
mentary discoid  placenta  is  first  formed,  as  in  the  rabbit ;  this 
gradually  spreads  over  the  whole  central  area,  till  only  the  ex- 
tremes (poles)  of  the  ovum  remain  free ;  villi  appear,  fitting  into 
pits  in  the  uterine  surface,  the  maternal  and  foetal  parts  of  the 
placenta  becoming  highly  vascular  and  closely  approximated. 
The  chorionic  zone  remains  wider  than  the  placental.  As  in 
man  there  is  at  birth  a  separation  of  the  maternal  as  well  as 
foetal  part  of  the  placenta — i.  e.,  the  latter  is  deciduate;  there  is 
also  the  beginning  of  a  decidua  reflexa. 

The  Diffuse  Placenta. — As  found  in  the  horse,  pig,  lemur, 
etc.,  the  allantois  completely  incloses  the  embryo,  and  it  be- 
comes villous  in  all  parts,  except  a  small  area  at  each  pole. 

The  Polycotyledonary  Placenta. — This  form  is  that  met  with 
in  ruminants,  in  wThich  case  the  allantois  completely  covers  the 
surface  of  the  subzonal  membrane,  the  placental  villi  being 
gathered  into  patches  (chorial  cotyledons),  which  are  equivalent 
to  so  many  independent  placentas.  The  component  villi  fit  into 
corresponding  pits  in  the  uterine  wall  (uterine  cotyledons), 
which  is  specially  thickened  at  these  points.  When  examined 
in  a  fresh  condition,  under  water,  they  constitute  very  beautiful 
objects.  The  pits  referred  to  above  into  which  the  foetal  villi  fit 
are,  as  shown  in  the  figures  on  page  91,  essentially  the  same  in 
structure  as  the  villi  themselves.  In  the  cow  the  uterine  cotyle- 
dons are  convex ;  but  in  the  sheep  and  goat  they  are  raised  con- 
cave cups  in  which  the  openings  for  the  foetal  villi  may  be  seen 
with  the  naked  eye.     The  differences  are  not  essential  ones. 

Between  the  uterine  cotyledons  and  the  foetal  villi  which 
fit  into  them  a  thickish,  milky-looking  fluid  is  found,  the 
"  uterine  milk  "  elaborated,  no  doubt,  by  the  cells  which  line  the 
cotyledonous  pits. 


90  COMPARATIVE  PHYSIOLOGY. 

The  placentation  of  certain  of  our  domestic  animals  may  be 
thus  expressed  in  tabular  form  (Fleming-) : 

Simple  placenta,  j  General-  1  s™- ' 

(  Local  and  circular,  i  Bitch. 

(  Cow. 

Multiple  placenta.  -j  Sheep. 

Comparing-  the  formation,  complete  development,  and  atro- 
phy (in  some  cases)  of  the  various  foetal  appendages  in  mam- 
mals, one  can  not  but  perceive  a  common  plan  of  structure, 
with  variations  in  the  preponderance  of  one  part  over  another 
here  arid  there  throughout.  In  birds  these  structures  are  sim- 
pler, chiefly  because  less  blended  and  because  of  the  presence 
of  much  food-yelk,  albumen,  egg-shell,  etc.,  on  the  one  hand, 
and  the  absence  of  a  uterine  wall,  with  which  in  the  mammal 
the  membranes  are  brought  into  close  relationship,  on  the  other ; 
but,  as  will  be  shown  later,  whatever  the  variations,  they  are 
adaptations  to  meet  common  needs  and  subserve  common  ends. 

MICROSCOPIC  STRUCTURE  OF  THE  PLACENTA. 

This  varies  somewhat  for  different  forms,  though,  in  that 
there  is  a  supporting  matrix,  minute  (capillary)  blood-vessels, 
and  epithelial  coverings  in  the  foetal  and  maternal  surfaces,  the 
several  forms  agree. 

The  pig  possesses  the  simplest  form  of  placenta  yet  known. 
The  villi  fit  into  depressions  or  crypts  in  the  maternal  uterine 
mucous  membrane.  The  villi,  consisting  of  a  core  of  connective 
tissue,  in  which  capillaries  abound,  are  covered  with  a  flat  epi- 
thelium; the  maternal  crypts  correspond,  being  composed  of 
a  similar  matrix,  lined  with  epithelium  and  permeated  by 
capillary  vessels,  which  constitute  a  plexus  or  mesh-work.  It 
thus  results  that  two  layers  of  epithelium  intervene  between 
the  maternal  and  foetal  capillaries. 

The  arrangement  is  substantially  the  same  in  the  diffuse  and 
the  cotyledonary  placenta. 

In  the  deciduate  placenta,  naturally,  there  is  greater  compli- 
cation. 

In  certain  forms,  as  in  the  fox  and  cat,  the  maternal  tissue 
shows  a  system  of  trabecular  assuming  a  meshed  form,  in 
which  run  dilated  capillaries.     These,  which  are  covered  with 


REPRODUCTION. 


91 


a  somewhat  columnar  epithelium,  are  everywhere  in  contact 
with  the  foetal  villi,  which  are  themselves  covered  with  a  flat 
epithelium. 


Fig.  94. 


Figs.  91  to  97. — Diagrammatic  representation  of  the  minute  structure  of  the  placenta 
(Foster  and  Balfour,  after  Turner).  F,  foetal;  M,  maternal  placenta;  e,  epithelium 
of  chorion;  e\  epithelium  of  maternal  placenta;  d,  foetal  blood-vessels;  d',  mater- 
nal blood-vessels;  v,  villus. 

Fig.  91. — Placenta  in  most  generalized  form. 

Fig.  92.— Structure  of  placenta  of  a  pig. 

Fiu.  93.— Of  a  cow. 

Fig.  94.— Of  a  fox. 

Fig.  95.— Of  a  cat. 

In  the  case  of  the  sloth,  with  a  more  discoidal  placenta,  the 
dilatation  of  capillaries  and  the  modification  of  epithelium  are 
greater. 


92 


COMPARATIVE  PHYSIOLOGY. 


In  the  placenta  of  the 
apes  and  of  the  human  sub- 
ject the  most  marked  depart- 
ure from  simplicity  is  found. 
The  maternal  vessels  are  said 
to  constitute  large  intercom- 
municating sinuses ;  the  villi 
may  hang  freely  suspended  in 
these  sinuses,  or  be  anchored 
to  their  walls  by  strands  of 
tissue.  There  is  believed  to 
be  only  one  layer  of  epithe- 
lial cells  between  the  vessels 
of  mother  and  foetus  in  the 
later  stages  of  pregnancy. 
This,  while  closely  investing 

Fig.  96.— Placenta  of  a  sloth.    Flat  maternal    the  f  oetal  vessels  (capillaries), 
epithelial  cells  shown  in   position   on  ...       ,     ,  ,       ,,  , 

right  side;  on  left  they  are  removed  and    really    belongs  to  the   mater- 

c£apu^ie^aexposedV.eSSel  wUh  itB  bl°°d"   nal  structures.     The  signifi- 


e-F. 


Fig  97.— Structure  of  human  placenta;  ds,  decidua  serotina;  L  trabecnlae  of  serotina 
passing  to  f«'lal  villi;  ca,  curling  artery;  up,  utero-placental  vein;  a;,  prolongation 
of  maternal  tissue  on  exterior  of  villus,  outside  cellular  layer  e',  which  may  repre- 
sent either  endothelium  of  maternal  blood-vessels  or  delicate  connective  tissue  of 
the  serotina  or  both;  e',  maternal  cells  of  the  serotina. 


REPRODUCTION.  93 

ca'nce  of  this  general  arrangement  will  be  explained  in  the 
chapter  on  the  physiological  aspects  of  the  subject. 

It  remains  to  inquire  into  the  relation  of  these  forms  to  one 
another  from  a  phylogenetic  (derivative)  point  of  view,  or  to 
trace  the  evolution  of  the  placenta. 

Evolution. — Passing  by  the  lowest  mammals,  in  which  the 
placental  relations  are  as  yet  imperfectly  understood,  it  seems 
clear  that  the  simplest  condition  is  found  in  the  rodentia. 
Thus,  in  the  rabbit,  as  has  been  described,  both  yelk-sac  and 
allantois  take  a  nutritive  part  ;  but  the  latter  remains  small. 
In  forms  above  the  rodents,  the  allantois  assumes  more  and 
more  importance,  becomes  larger,  and  sooner  or  later  predomi- 
nates over  the  yelk-sac. 

The  discoidal,  zonary,  cotyledonary,  etc.,  are  plainly  evolu- 
tions from  the  diffuse,  for  both  differentiation  of  structure  and 
integration  of  parts  are  evident.  Tbe  human  placenta  seems 
to  have  arisen  from  the  diffuse  form ;  and  it  will  be  remembered 
that  it  is  at  one  period  represented  by  the  chorion  with  its  villi 
distributed  universally. 

The  resemblance  of  the  embryonic  membranes  at  any  early 
stage  in  man  and  other  mammals  to  those  of  birds  certainly 
suggests  an  evolution  of  some  kind,  though  exactly  along  what 
lines  that  has  taken  place  it  is  difficult  to  determine  with  exact- 
ness ;  however,  as  before  remarked,  nearly  all  the  complica- 
tions of  the  higher  forms  arise  by  concentration  and  fusion,  on 
the  one  hand,  and  atrophy  and  disappearance  of  parts  once 
functionally  active,  on  the  other. 

Summary. — The  ovum  is  a  typical  cell ;  unspecialized  in  most 
directions,  but  so  specialized  as  to  evolve  from  itself  compli- 
cated structures  of  higher  character.  The  segmentation  of  the 
ovum  is  usually  preceded  by  fertilization,  or  the  union  of  the 
nuclei  of  male  and  female  cells,  which  is  again  preceded  by  the 
extrusion  of  polar  globules.  In  the  early  changes  of  the  ovum, 
including  segmentation,  periods  of  rest  and  activity  alternate. 
The  method  of  segmentation  has  relation  to  the  quantity  and 
arrangement  of  the  food-yelk.  Ova  are  divisible  generally 
into  completely  segmenting  (holoblastic),  and  those  that  under- 
go segmentation  of  only  a  part  of  their  substance  (meroblastic)  ; 
but  the  processes  are  fundamentally  the  same. 

Provision  is  made  for  the  nutrition,  etc.,  of  the  ovum,  when 
fertilized  (oosperm)  by  the  formation  of  yelk-sac  and  allan- 
tois; as  development  proceeds,  one  becomes  more  prominent 


94  COMPARATIVE  PHYSIOLOGY. 

than  the  other.  The  allantois  may  fuse  with  adjacent  mem- 
branes and  form  at  one  part  a  condensed  and  hypertrophied 
chorion  (placenta),  with  corresponding1  atrophy  elsewhere. 
The  arrangement  of  the  placenta  varies  in  different  groups  of 
animals  so  constantly  as  to  furnish  a  basis  for  classification. 
Whatever  the  variations  in  the  structure  of  the  placenta,  it  is 
always  highly  vascular ;  its  parts  consist  of  villi  fitting  into 
crypts  in  the  maternal  uterine  membrane — both  the  villi  and 
the  crypts  being  provided  with  capillaries  supported  by  a  con- 
nective-tissue matrix  covered  externally  by  epithelium.  The 
placenta  in  its  different  forms  would  appear  to  have  been 
evolved  from  the  diffuse  type. 

The  peculiarities  of  the  embryonic  membranes  in  birds  are 
owing  to  the  presence  of  a  large  food-yelk,  egg-shell,  and  egg- 
mernbranes ;  but  throughout,  vertebrates  follow  in  a  common 
line  of  development,  the  differences  which  separate  them  into 
smaller  and  smaller  groups  appearing  later  and  later.  The 
same  may  be  said  of  the  animal  kingdom  as  a  whole.  This 
seems  to  point  clearly  to  a  common  origin  with  gradual  diver- 
gence of  type. 


THE  DEVELOPMENT  OF  THE   EMBRYO  ITSELF. 


We  now  turn  to  the  development  of  the  body  of  the  animal 
for  which  the  structures  we  have  been  describing  exist.  It  is 
important,  however,  to  remember  that  the  development  of 
parts,  though  treated  separately  for  the  sake  of  convenience, 
really  goes  on  together  to  a  certain  extent;  that  new  structures 
do  not  appear  suddenly  but  gradually ;  and  that  the  same  law 
applies  to  the  disappearance  of  organs  which  are  being  super- 
seded by  others.  To  represent  this  completely  would  require 
lengthy  descriptions  and  an  unlimited  number  of  cuts;  but 
with  the  above  caution  it  is  hoped  the  student  may  be  able  to 
avoid  erroneous  conceptions,  and  form  in  his  own  mind  that 
series  of  pictures  which  can  not  be  well  furnished  in  at  least 
the  space  we  have  to  devote  to  the  subject.  But,  better  than 
any  abstract  statements  or  pictorial  representations,  would  be 
the  examination  of  a  setting  of  eggs  day  by  day  during  their 
development  under  a  hen.  This  is  a  very  simple  matter,  and, 
while  the  making  and  mounting  of  sections  from  hardened 
specimens  is  valuable,  it  may  require  more  time  than  the 
student  can  spare ;  but  it  is  neither  so  valuable  nor  so  easily  ac- 
complished as  what  we  have  indicated;  for,  while  the  lack  of 
sections  made  by  the  student  may  be  made  up  in  part  by  the 
exhibition  to  him  of  a  set  of  specimens  permanently  mounted 
or  even  by  plates,  nothing  can,  in  our  opinion,  take  the  place 
of  the  examination  of  eggs  as  we  have  suggested.  It  prepares 
for  the  study  of  the  development  of  the  mammal,  and  exhibits 
the  membranes  in  a  simplicity,  freshness,  and  beauty  which 
impart  a  knowledge  that  only  such  direct  contact  with  nature 
can  supply.  To  proceed  with  great  simplicity  and  very  little 
apparatus,  one  requires  but  a  forceps,  a  glass  dish  or  two,  a 
couple  of  watch-glasses,  or  a  broad  section-lifter  (even  a  case- 
knife  will  answer),  some  water,  containing  just  enough  salt  to 
be  tasted,  rendered  lukewarm  (blood-heat). 


96 


COMPARATIVE   PHYSIOLOGY. 


Holding  the  egg  longitudinally,  crack  it  across  the  center 
transversely,  gently  ahd  carefully  pick  away  the  shell  and  its 


Fig.  98.— Various  stages  in  the  development  of  the  frog  from  the  egg  (after  Howes). 
1.  The  segmenting  ovum,  showing  first  cleavage  furrow.  2.  Section  of  the  above 
at  right  angles  to  the  furrow.  3.  Same,  on  appearance  of  second  furrow,  viewed 
slightly  from  above.  4.  The  latter  seen  from  beneath.  5.  The  same,  on  appear- 
ance of  first  horizontal  furrow.  6.  The  same,  seen  from  above.  7.  Longitudinal 
section  of  6.  8  and  9.  Two  phases  in  segmentation,  on  appearance  of  fourth  and 
fifth  furrows.  10.  Longitudinal  vertical  section  at  a  slightly  later  stage  than  the 
above.  11.  Later I stage.  Upper  pigmented  pole  dividing  more  rapidly  than  lower. 
12.  Later  phase  of  11.  13.  Longitudinal  vertical  section  of  12.  14.  Segmenting 
ovum  at  blastopore  stage.  15.  Longitudinal  vertical  section  of  same.  13  and  IB 
•a  10  (all  others  x  5).  10.  Longitudinal  vertical  section  of  embryo  at  a  stage  later 
than  14d  x  10).  nc,  nucleus;  c.  c,  cleavage  cavity;  eg,  epiblast;  1. 1,  yelk-Bearing 
lower-layer  cells;  bl,  blastopore;  al,  archentcron  (mid-gut);  hb,  hypoblast;  wis, 
undifferentiated  mesoblast;  ch,  notochord;  n.  a,  neural  (cerebrospinal)  axis. 

membranes,  when  the  blastoderm  may  be  seen  floating  upward, 
as  it  always  does.      It  should  be  well   examined   in   position, 


THE  DEVELOPMENT   OF   THE   EMBRYO  ITSELF.      97 


using  a  hand  lens,  though  this  is 
knowledge ;  in  fact,  if  the  exam- 
ination goes  no  further  than  the 
naked-eye  appearances  of  a  dozen 
eggs,  selecting  one  every  twenty- 
four  hours  during  incubation, 
when  opened  and  the  shell  and 
membranes  well  cleared  away, 
such  a  knowledge  will  be  sup- 
plied as  can  be  obtained  from  no 
books  or  lectures  however  good. 
It  will  be,  of  course,  understood 
that  the  student  approaches  these 
examinations  with  some  ideas 
gained  from  plates  and  previous 
reading.  The  latter  will  furnish 
a  sort  of  biological  pabulum  on 
which  he  may  feed  till  he  can 
supply  for  himself  a  more  natu- 
ral and  therefore  more  healthful 
one.  While  these  remarks  apply 
with  a  certain  degree  of  force  to 
all  the  departments  of  physiolo- 
gy, they  are  of  special  impor- 
tance to  aid  the  constructive  fac- 
ulty in  building  up  correct  no- 
tions of  the  successive  rapid 
transformations  that  occur  in 
the  development  of  a  bird  or 
mammal. 

Fig.  99  shows  the  embryo  of 
the  bird  at  a  very  early  period, 
when  already,  however,  some  of 
the  main  outlines  of  structure 
are  marked  out.  Development 
in  the  fowl  is  so  rapid  that  a  few 
days  suffice  to  outline  all  the 
principal  organs  of  the  body.  In 
the  mammal  the  process  is  slow- 
er, but  in  the  main  takes  place  in 
the  same  fashion. 

As  the  result  of  long  and  pa- 
7 


not  essential  to  getting  a  fair 


Fig.  99.— Embryo  fowl  3  mm.  long,  of 
about  twenty-four  hours,  seen  from 
above.  1  x  39.  (Haddon,  after 
K611iker.)  Mn,  union  of  the  med- 
ullary folds  in  the  region  of  the 
hind-brain;  Pr,  primitive  streak; 
Pz,  parietal  zone  ;  7?/,  posterior 
portion  of  widely  open  neural 
groove;  i?/'.  anterior  part  of  neu- 
ral groove  ;  Rw,  neural  ridge  ;  Stz. 
trunk-zone  ;  rAf.  anterior  amni 
otic  fold  ;  rD,  anterior  umbilical 
sinus  showing  through  the  blasto- 
derm. His  divides  the  embryonic 
rudiment  into  a  central  trunk-zone 
and  a  pair  of  lateral  or  parietal 
zones. 


98 


COMPARATIVE   PHYSIOLOGY. 


tient  observation,  it  is  now  settled  that  all  the  parts  of  the  most 
complicated  organism  arise  from  the  three-layered  blastoderm 
previously  figured  ;  every  part  may  be  traced  back  as  arising  in 
one  or  other  of  these  layers  of  cells — the  epiblast,  mesoblast,  or 
hypoblast.  It  frequently  happens  that  an  organ  is  made  up  of 
cells  derived  from  more  than  one  layer.  Structures  may,  ac- 
cordingly, be  classified  as  epiblastic,  mesoblastic,  or  hypoblastic ; 
for,  when  two  strata  of  cells  unite  in  the  formation  of  any  part, 
one  is  always  of  subordinate  importance  to  the  other :  thus  the 
digestive  organs  are  made  up  of  mesoblast  as  well  as  hypo- 
blast, but  the  latter  constitutes  the  essential  secreting  cell  mech- 
anism. As  already  indicated,  the  embryonic  membranes  are 
also  derived  from  the  same  source. 

The  epiblast  gives  rise  to  the  skin  and  its  appendages  (hair, 
nails,  feathers,  etc.),  the  whole  of  the  nervous  system,  and  the 
chief  parts  of  the  organs  of  special  sense. 

The  mesoblast  originates  the  vascular  system,  the  skeleton, 
all  forms  of  connective  tissue  including  the  framework  of 
glands,  the  muscles,  and  the  epithelial  (endothelial)  structures 
covering  serous  membranes. 

The  hypoblast  furnishes  the  secreting  cells  of  the  digestive 
tract  and  its  appendages — as  the  liver  and  pancreas— the  lining 
epithelium  of  the  lungs,  and  the  cells  of  the  secreting  mucous 
membranes  of  their  framework  of  bronchial  tubes. 

It  is  difficult  to  overrate  the  importance  of  these  morpholog- 
ical generalizations  for  the  physiologist  ;  for,  once  the  origin  of 
an  organ  is  known,  its  function  and  physiological  relations  gen- 
erally may  be  predicted  with  considerable  certainty.     We  shall 


Pig.  100.— Transverse  section  through  the  medullary  groove  and  half  the  blastoderm 
of  a  chick  of  eighteen  hours  (Foster  and  Balfour).  E,  epiblast;  M,  mesoblast; 
//,  hypoblast;  mf',  medullary  fold;  my,  medullary  groove;  ch,  notochord. 

endeavor  to  make  this  prominent  in  the  future  chapters  of  this 
work. 

Being  prepared  with  these  generalizations,  we  continue  our 
study  of  the  development  of  the  bird's  embryo.     Before  the  end 


THE   DEVELOPMENT  OP   THE  EMBRYO  ITSELF.      99 

of  the  first  twenty-four  hours  such  an  appearance  as  that  repre- 
sented in  Fig.  100  is  presented. 

The  mounds  of  cells  forming  the  medullary  folds  are  seen 
coming  in  contact  to  form  the  medullary  {neural)  canal. 


Fig.  101.— Transverse  section  of  embryo  chick  at  end  of  first  day  (after  KOlliker).  M, 
mesoblast;  H.  hypoblast;  m,  medullary  plate;  E.  epiblast;  m.  g,  medullary  groove; 
m.f,  medullary  fold;  ch,  chorda  dorsalis  ;  P,  protovertebral  plate;  d.  in.  division 
of  mesoblast. 

The  notochord,  marking  out  the  future  bony  axis  of  the 
body,  may  also  be  seen  during  the  first  day  as  a  well-marked 
linear  extension,  just  beneath  the  medullary  groove.    The  cleav- 


nf    fif 


Fig.  102. — Transverse  section  of  chick  at  end  of  second  day  (KOlliker).  E,  epiblast; 
H,  hypoblast;  e.  m,  external  plate  of  mesoblast  dividing  (cleavage  of  mesoblast): 
m.f,  medullary  fold;  m.  rj,  medullary  groove;  ao,  aorta;  p,  pleuroperitoneal  cavity; 
P,  protovertebral  plate. 

age  of  the  mesoblast,  resulting  in  the  commencement  of  the 
formation  of  somatopleure  (body -fold)  and  the  splanchnopleure 
(visceral  fold),  is  also  an  early  and  important  event.  These 
give  rise  between  them  to  the  pleuro-peritoneal  cavity .  The 
portions  of  mesoblast  nearest  the  neural  canal  form  masses  (ver- 
tebral plates)  distinct  from  the  thinner  outer  ones  (lateral 
plates).  The  vertebral  plates,  when  distinctly  marked  off,  as 
represented  in  the  figure,  are  termed  the  protovertebral  (meso- 
blastic  somites),  and  represent  the  future  vertebra?  and  the  vol- 
untary muscles  of  the  trunk ;  the  former  arising  from  the  inner 
subdivisions,  and  the  latter  from  the  outer  (muscle-plates).  It 
will  be  understood  that  the  protovertebra?  are  the  results  of 


100 


COMPARATIVE   PHYSIOLOGY. 


m.b 


au.p, 


a.p. 


transverse  division  of  the  columns  of  mesoblast  that  formed  the 
vertebral  plates. 

Before  the  permanent  vertebrae  are  formed,  a  reunion  of  the 
original  pro  to  vertebrae  takes  place  as  one  cartilaginous  pillar, 

followed  by  a  new  segmen- 
tation midway  between  the 
original  divisions. 

It  is  thus  seen  that  a 
large  number  of  structures 
either  appear  or  are  clearly 
outlined  during  the  first 
day  of  incubation  :  the 
primitive  streak,  primitive 
groove,  medullary  plates 
and  groove,  the  neural  ca- 
nal, the  head-fold,  the 
cleavage  of  the  mesoblast, 
the  protovertebrae.  with 
traces  of  the  amnion  and 
area  opaca. 

During  the  second  day 
nearly  all  the  remaining 
important  structures  of  the 
chick  are  marked  out,  while 
those  that  arose  during  the 
first  day  have  progressed. 
Thus,  the  medullary  folds 
close  ;  there  is  an  increase 
in  the  number  of  protover- 
tebrae ;  the  formation  of  a 
tubular  heart  and  the  great 
blood-vessels  ;  the  appear- 
ance of  the  Wolffian  duct  ; 
the  progress  of  the  head  re- 
gion ;  the  appearance  of  the 
three  cerebral  vesicles  at  the  anterior  extremity  of  the  neural 
canal  ;  the  subdivision  of  the  first  cerebral  vesicle  into  the  optic 
vesicles  and  the  beginnings  of  the  cerebrum  ;  the  auditory  pit 
arising  in  the  third  cerebral  vesicle  (hind-brain) ;  cranial  flex- 
ure commences  ;  both  head  and  tail  folds  become  more  dis- 
tinct ;  the  heart  is  not  only  formed,  but  its  curvature  becomes 
more  marked  and  rudiments  of  auricles  arise  ;   while  outside 


Fig.  103. — Embryo  of  chick,  between  thirty 
and  thirty-six  hours,  viewed  from  above 
as  an  opaque  object  (Foster  and  Balfour). 
/.  h,  forebrain;  m.  b,  midbrain;  h.  b,  hind- 
brain;  op.  v,  optic  vesicle;  au.  p,  auditory 
pit  ;  o.f.  vitelline  vein  ;  p.  v,  mesoblastic 
somite';  m.f,  line  of  function  of  medulla- 
ry folds  abovejmedullary  canal ;  s.r,  sinus 
raomboidalis;  I,  tail-fold;  p.r,  remains  of 
primitive  groove  ;  a.p,  area  pellucida. 


THE   DEVELOPMENT   OP  THE   EMBRYO   ITSELF.    101 


the  embryo  itself  the  circulation  of  the  yelk-sac  is  established, 
the  allantois  originates,  and  the  amnion  makes  rapid  progress. 

It  may  be  noticed  that  the  cerebral  vesicles,  the  optic  vesi- 
cles, and  the  auditory  pit  are  all  derived  from  the  epiblastic 
accumulations  which  occur  in  the  anterior  extremity  of  the 
embryo  ;  and  their  early  appearance  is  prophetic  of  their  physi- 
ological importance. 

The  heart,  too,  so  essential  for  the  nutrition  of  the  embryo, 
by  distributing  a  constant  blood-stream,  is  early  formed,  and 
becomes  functionally  active.  It  arises  beneath  the  hind-end  of 
the  fore-gut,  at  the  point  of  divergence  of  the  folds  of  the 


B     __ 


Fig.  104. — Diagram  representing  under  surface  of  an  embryo  rabbit  of  nine  days  and 
three  hours  old,  illustrating  development  of  the  heart  (after  Allen  Thomson).  A. 
view  of  the  entire  embryo;  B,  an  enlarged  outline  of  the  heart  of  A;  C.  later  stage 
of  the  development  of  B ;  h  h,  ununited  heart;  aa,  aorUe;  vv,  vitillme  veins. 

splanchnopleure,  and  so  lies  within  the  pleuro-peritoneal  cav- 
ity, and  is  derived  from  the  mesoblast.  At  the  beginning  the 
heart  consists  of  two  solid  columns  ununited  in  front  at  first  ; 
later,  these  fuse,  in  part,  so  that  they  have  been  compared 
with  an  inverted  Y,  in  which  the  heart  itself  would  correspond 
to  the  lower  stem  of  the  letter  Q)  and  the  great  veins  (vitel- 
line) to  its  main  limbs.     The  solid  cords  of  mesoblast  become 


102 


COMPARATIVE  PHYSIOLOGY. 


hollow  prior  to  their  coalescence,  when  the  two  tubes  become 
one. 

The  entire  blood-vascular  system  originates  in  the  mesoblast 
of  the  area  opaca  especially ;  at  first  appearing  in  isolated  spots 


Pro.  105.— Chick  on  third  day,  seen  from  beneath  as  a  transparent  object,  the  head 
being  turned  to  one  side  (Foster  and  Balfour),  a',  false  amnion;  a,  amnion;  OH, 
cerebral  hemisphere;  FB,  MB,  HB,  anterior,  middle,  and  posterior  cerebral  vesi- 
cles; OP,  optic  vesicle;  ot,  auditory  vesicle;  OfV,  omphalo-mesentcric  veins;  lit, 
heart;  Ao,  bulbus  arteriosus;  ch,  notochord;  Of.a,  omphalomesenteric  arteries; 
Pv,  proto vertebra;;  x,  point  of  divergence  of  the  splanchnopleural  folds;  y,  ter- 
mination of  the  fore-gut,  V. 

which  come  together  as  actual  vessels  are  formed.  The  student 
who  will  pursue  the  plan  of  examining  a  series  of  incubating 
eggs  will  be  struck  with  the  early  rise  and  rapid  progress  of  the 
vascular  system  of  the  embryo,  which  takes,  when  complete, 
such  a  form  as  is  represented  diagramatically  in  Fig.  109. 

The  blood  and  the  blood-vessels  arise  simultaneously  from 


THE   DEVELOPMENT   OF  THE  EMBRYO  ITSELF.    103 


the  cells  of  the  mesoblast  by  outgrowths  of  nuclear  prolifera- 
tion, and  in  the  case  of  vessels  (Fig.  143)  extension  of  processes, 
fusion,  and  excavation. 


Fig.  106.— Diagram  of  the  heart  and  principal  arteries  of  the  chick  (Qnain).  A  repre- 
sents an  earlier  and  B  and  C  later  stages.  1, 1,  omphalo-mesenteric  reins;  2,  auri- 
cle; 3,  ventricle;  4,  aortic  bulb;  5,5,  primitive  aortse;  6,6,  omphalo-mesenteric 
arteries;  A,  united  aortse. 

The  fore-gut  is  formed  by  the  union  of  the  folds  of  the 
splanchnopleure  from  before  backward,  and  the  hind-gut  in  a 
similar  manner  by  fusion  from  behind  forward. 


Fig.  107. — Diagrammatic  outlines  of  the  early  arterial  system  of  the  mammalian  em- 
bryo (after  Allen  Thomson).  A.  At  a  period  corresponding  to  the  thirty-sixth  or 
thirty-eighth  hour  of  incubation.  B.  Later  stage,  with  two  pairs  of  aortic  arches. 
/>.  bulbus  arteriosus  of  heart;  v,  vitelline  arteries;  1 — 5,  the  aortic  arches.  The 
dotted  lines  indicate  the  position  of  the  future  arches. 


104 


COMPARATIVE   PHYSIOLOGY. 


The  excretory  system  is  also  foreshadowed  at  an  early  period 
the  Wolffian  duct  (Fig.  110),  a  mass  of  mesoblast  cells  near 
which  the  cleavage  of  the  mesoblast 
takes  place. 

During  the  latter  part  of  the  sec- 
ond day  the  vascular  system,  includ- 
ing the  heart,  makes  great  progress. 
The  latter,  in  consequence  of  excessive 
growth  and  the  alteration  of  the  rela- 
tive position  of  other  parts,  becomes 
bent  up  on  itself,  so  that  it  presents 
a  curve  to  the  right  which  represents 
the  venous  part,  and  one  to  the  left, 
answering  to  the  arterial.  The  rudi- 
ments of  the  auricles  also  are  to  be 
seen. 

The  arterial  system  is  represented 
at  this  stage  by  the  expanded  portion 
of  the  heart  known  as  the  bulbus  ar- 
teriosus, and  two  extensions  from  it, 
the  aorta?,  which,  uniting  above  the 
alimentary  canal,  form  a  single  poste- 
rior or  dorsal  aorta.  From  these  great 
arterial  vessels  the  lesser  ones  arise, 
and  by  subdivision  constitute  that 
great  mesh-work  represented  cliagram- 
matically  in  Figs.  108, 109,  from  which 
,.  the  course  of  the  circulation  may  be 
bryonic    vascular    system   o-athered.      The  beating  of  the  heart 

(Wiedersheim).    A,  atrium;    °  0 

A',  A',  dorsal  aorta ;    Ab,    commences  before  the  corpuscles  nave 

branchial  vessels:  Acd,  cau-    1  ,       i  -i     ,-i„  .    -i 

dai  artery;  All,  allantoic  (hy-  become  numerous,  and  while  the  tub- 
fSSFStSSfi  l^bus  ular  system,  through  which  the  blood 
arteriosus,-  c,  c>.  external  and   js  ^0  be  driven,  is  still  very  incomplete. 

internal  carotids;  D,  ductus  '  .  *■ 

The  events  of  the  third  day  are  of 


Cuvieri  (precaval  veins);  E, 
external  iliac  arteries;  H.  G, 
posterior  cardinal  vein ;  Ic, 
common  iliac  arteries;  If.  L. 
Kill  clefts  ;  R.  A,  right  and 
left,  rootn  of  the  aorta;  S.  S', 
branchial  collecting  trunks 
or  veins  ;  ,S7>.  subclavian  ar- 
tery ;  8b',  subclavian  vein; 
Si,  sinus  venosus;  V,  ventri- 
cle ;  VC,  anterior  cardinal 
vein;   Vm,  vitelline  veins. 


left  side 


the  nature  of  the  extension  of  parts 
already  marked  out  rather  than  the 
formation  of  entirely  new  ones.  The 
following  are  the  principal  changes  : 
The  bending  of  the  head-end  down- 
ward (cranial  flexure)  ;  the  turning 
of  the  embryo  so  that  it  lies  on  its 
the  completion  of  the  vitelline  circulation  ;  the  in- 


THE  DEVELOPMENT  OF  THE  EMBRYO  ITSELF.    105 

crease  in  the  curvature  of  the  heart  and  its  complexity  of  struct- 
ure by  divisions  ;  the  appearance  of  additional  aortic  arches 
and  of  the  cardinal  veins  ;  the  formation  of  four  visceral  clefts 
and  five  visceral  arches  ;  a  series  of  progressive  changes  in 


AAA 


Fig.  109.— Diagram  of  circulation  of  yelk-sac  at  end  of  third  day  (Foster  and  Bal- 
four). Blastoderm  seen  from  below.  Arteries  made  black.  H,  heart;  AA,  sec- 
ond, third,  and  fourth  aortic  arches;  A  0,  dorsal  aorta;  L.of.A,  left  vitelline 
artery;  E.qf.  A.  right  vitelline  artery;  8.  T.  sinus  terminalis;  'L.  of,  left  vitelline 
vein;   R.  of,  right  vitelline  vein ;   S.  V,  sinus  venosus  ;  D.  C,  ductus 


S.  Ca.  V,  superior  cardinal  or  jugular  vein;   V.  Ca,  inferior  cardinal  vein, 


Cuvicri; 


the  organs  of  the  special  senses,  such  as  the  formation  of  the 
lens  of  the  eye  and  a  secondary  optic  vesicle  ;  the  closing  in  of 
the  optic  vesicle  ;  and  the  formation  of  the  nasal  pits.  In  the 
region  of  the  future  brain,  the  vesicles  of  the  cerebral  hemi- 
spheres become  distinct ;  the  hind-brain  separates  into  cei'e- 


106 


COMPARATIVE  PHYSIOLOGY. 


!    ;/p?^ 


Fio.  111. 


Fio.  112. 


^VaWFosTe^nTttX!^  through '^'^r  remon  of  an  embryo  at  end  of  fourth 
aay i t  osier  and  Balfour)  n.  c,  neural  canal;  pr,  posterior  root  of  spinal  nerve 
A  VT&of^hS^  r°0t-;  A-  °-  °'  !,l",erior  ^y  column  of  spinal  cord; 
notochor.i  I  v  ,r  U-T  ln  ^urse  ot ■formation;  m.p,  muscle-plate;  c.h, 
vPin-  w  /  w7f;rW0lhu'.'  r$P,'-  A0'  dorsal  aorta;  »■«■«,  Posterior  cardina 
vein;  W.  d,  Wolffian  duct;   W.  b,  Wolffian  body,  consisting  of  tubules  and  Mat 


THE   DEVELOPMENT   OF   THE  EMBRYO    ITSELF.   107 

pighian  corpuscles;  g.  e,  germinal  epithelium;  d,  alimentary  canal;  M,  commenc- 
ing mesentery  ;  S.  0,  somatopleure  ;  SP,  splanchnopleure  ;  V,  blood  -  vessels  ; 
pp,  pleuroperitoneal  cavity. 

Fig.  111.— Diagram  of  portion  of  digestive  tract  of  chick  on  fourth  day  (after  GOtte). 
The  black  line  represents  hypoblast;  the  shaded  portion,  mesoblast;  Iff,  lung  di- 
verticulum, expanding  at  bases  into  primary  lung  vesicle;  st,  stomach;  /,hver; 
p,  pancreas. 

Fig.  112.— Head  of  chick  of  third  day,  viewed  sidewise  as  a  transparent  object  (Hux- 
ley). /«,  cerebral  hemispheres;  lb,  vesicle  of  third  ventricle;  II,  mid-brain;  III, 
hind-brain;  a,  optic  vesicle;  cj,  nasal  pit;  b,  otic  vesicle;  d,  infundibulum  ;  e, 
pineal  body ;  h,  notochord  ;  V,  fifth  nerve  ;  VII,  seventh  nerve  ;  VIII,  united 
glossopharyngeal  and  pneumogastric  nerves.     1,  2,  3,  4,  5,  the  five  visceral  folds. 

bellum  and  medulla  oblongata  ;  the  nerves,  both  cranial  and 
spinal,  bud  out  from  the  nervous  centers.  The  alimentary  ca- 
nal enlarges,  a  fore-gut  and  hind-gut  being  formed,  the  former 
being  divided  into  oesophagus,  stomach,  and  duodenum  ;  the 
latter  into  the  large  intestine  and  the  cloaca.  The  lungs  arise 
from  the  alimentary  canal  in  front  of  the  stomach  ;  from  simi- 
lar diverticula  from  the  duodenum,  the  liver  and  pancreas  orig- 
inate. Changes  in  the  protovertebrae  and  muscle-plates  con- 
tinue, while  the  Wolffian  bodies  are  formed  and  the  Wolffian 
duct  modified. 

Up  to  the  third  day  the  embryo  lies  mouth  downward,  but 
now  it  comes  to  lie  on  its  left  side.  See  Fig.  105  with  the  ac- 
companying description,  it  being  borne  in  mind  that  the  view  is 
from  below,  so  that  the  right  in  the  cut  is  the  left  in  the  em- 

FrB. 


Fig.  113.— Head  of  chick  of  fourth  day.  viewed  from  below  as  an  opaque  object  (Fos- 
ter and  Balfour).  The  neck  is  cut  across  between  third  and  fourth  visceral  folds. 
C.  //,  cerebral  hemispheres;  F.  B,  vesicle  of  third  ventricle:  Op,  eyeball;  /if. 
naso-frontal  process;  M,  cavity  of  mouth;  S.  J/,  superior  maxillary  process  of  F. 
1.  the  first  visceral  fold  (mandibular  arch);  F.  2,  F.  3,  second  and  third  visceral 
arches;  JV,  nasal  pit. 

bryo  itself.  Fig.  110  gives  appearances  furnished  by  a  vertical 
transverse  section.  The  relations  of  the  parts  of  the  digestive 
tract  and  the  mode  of  origin  of  the  lungs  may  be  learned  from 
Fig.  111. 


108 


COMPARATIVE   PHYSIOLOGY. 


An  examination  of  the  figures  and  subjoined  descriptions 
must  suffice  to  convey  a  general  notion  of  the  subsequent  prog- 


G.Ph 


VII.        rV.V.  IF 


MPr 


Fig.  114.— Embryo  at  end  of  fourth  day,  seen  as  a  transparent  object  (Foster  and 
Balfour).  C'H,  cerebral  hemisphere;  F.  B.  fore-brain,  or  vesicle  of  third  ventricle 
(thalamencephalon),  with  pineal  gland  (Pn)  projecting;  M.  B,  mid-brain;  C.b, 
cerebellum;  IV.  V,  fourth  ventricle;  L,  lens;  cJw,  choroid  slit;  Cen.  V,  auditory 
vesicle;  sm,  superior  maxillary  process;  IF,  2F,  etc.,  first,  second,  etc.,  visceral 
folds ;  V,  fifth  nerve;  VII.  seventh  nerve;  O.  Ph,  glossopharyngeal  nerve;  Pg, 
pneumogastric.  The  distribution  of  these  nerves  is  also  indicated  ;  ch,  noto- 
chord;  lit,  heart;  MP.  muscle-plates;  W,  wing;  H.  L,  hind-limb.  The  amnion 
has  been  removed.    Al,  allantois  protruding  from  cut  end  of  somatic  stalk  SS. 

ress  of  the  embryo.  Special  points  will  be  considered,  either  in 
a  separate  chapter  now,  or  deferred  for  treatment  in  the  body 
of  the  work  from  time  to  time,  as  they  seem  to  throw  light 
upon  the  subjects  under  discussion. 


DEVELPOMENT  OF  THE  VASCULAR  SYSTEM  IN  VER- 
TEBRATES. 

This  subject  has  been  incidentally  considered,  but  it  is  of 
such  importance  morphological,  physiological,  and  pathological, 
as  to  deserve  special  treatment. 

In  the  earliest  stages  of  the  circulation  of  a  vertebrate  the 
arterial  system  is  made  up  of  a  pair  of  arteries  derived  from  the 
single  bulbus  arteriosus  of  the  heart,  which,  after  passing  for- 


THE   DEVELOPMENT   OP   THE   EMBRYO   ITSELF.    109 


ward,  bends  round  to  the  dorsal  side  of  the  pharynx,  each  giving 
off  at  right  angles  to  the  yelk-sac  a  vitelline  artery  ;  the  aorta? 
unite  dorsally,  then  again  separate  and  become  lost  in  the  pos- 
terior end  of  the  embryo.  The  so-called  arches  of  the  aorta  are 
large  branches  in  the  anterior  end  of  the  embryo  derived  from 
the  aorta  itself. 

The  venous  system  corresponding  to  the  above  is  composed 
of  anterior  and  posterior  pairs  of  longitudinal  (cardinal)  veins, 
the  former  (jugular,  cardinal)  uniting  with  the  posterior  to  form 
a  common  trunk  (ductus  Cuvieri)  by  which  the  venous  blood  is 
returned  to  the  heart.  The  blood  from  the  posterior  part  of  the 
yelk-sac  is  collected  by  the  vitelline  veins,  which  terminate  in 
the  median  sinus  venosus. 

The  Later  Stages  of  the  Foetal  Circulation.— Corresponding 
to  the  number  of  visceral  arches  five  pans  of  aortic  arches  arise ; 
but  they  do  not  exist  together,  the  first  two  having  undergone 
more  or  less  complete  atrophy  before  the  others  appear.  Figs. 
115, 116  convey  an  idea  of  how  the  permanent  forms  (indicated  by 
darker  shading)  stand  related  to  the  entire  system  of  vessels  in 
different  groups  of  animals.  Thus,  in  birds  the  right  (fourth) 
aortic  arch  only  remains  in  connection  with  the  aorta,  the  left 
forming  the  subclavian  artery,  while  the  reverse  occurs  in 
mammals.  The  fifth  arch  (pulmonary)  always  supplies  the 
lungs. 


Fig.  115.— Diagrams  of  the  aortic  arches  of  mammal  (Landois  and  Stirling,  after 
Ratlike).  1.  Arterial  trunk  with  one  pair  of  arches,  and  an  indication  where  the 
second  and  third  pairs  will  develop.  2.  Ideal  Btagc  of  five  complete  arches:  the 
fourth  clefts  are  shown  on  the  left  side.  3.  The  two  anterior  pairs  of  arches  have 
disappeared.  -1.  Transition  to  the  final  stage.  A,  aortic  arch;  ad,  dorsal  aorta; 
ax,  subclavian  or  axillary  artery;  ft.  external  carotid;  Ci.  internal  carotid;  dB, 
ductus  arteriosus  Botalli;  P.  pulmonary  artery;  iff,  subclavian  artery;  la,  truncua 
arteriosus;  v,  vertebral  artery. 

The  arrangement  of  the  principal  vessels  in  the  bird,  mam- 
mal, etc. ,  is  represented  on  page  110.     In  mammals  the  two  prim- 


110 


COMPARATIVE   PHYSIOLOGY. 


itive  anterior  abdominal  {allantoic)  veins  develop  early  and 
unite  in  front  with  the  vitelline :  hut  the  right  allantoic  vein 
and  the  right  vitelline  veins  soon  disappear,  while  the  long  com- 
mon trunk  of  the  allantoic  and  vitelline  veins  {ductus  venosus) 
passes  through  the  liver,  where  it  is  said  the  ductus  venosus 
gives  off  and  receives  branches.  The  ductus  venosus  Arantii 
persists  throughout  life.  (Compare  the  various  figures  illustrat- 
ing the  circulation.) 

A         ...  ..         B 


Fro.  116.— Diagram  illustrating  transformations  of  aortic  arches  in  a  lizard,  A  ;  a 
snake,  B;  a  bird.  C;  a  mammal,  D.  Seen  from  below,  (lladdon,  after  Rathke.) 
a,  internal  carotid;  b,  external  carotid;  c,  common  carotid.  A.  d,  ductus  Botalli 
between  the  third  and  fourth  arches;  e,  right  aortic  arch;  /,  subclavian;  g,  dorsal 
aorta;  h,  left  aortic  arch;  i,  pulmonary  artery;  k,  rudiment  of  the  ductus  Botalli 
between  the  pulmonary  artery  and  the  aortic  arches.  B.  d,  right  aortic  arch;  e, 
vertebral  artery;  /,  left  aortic  arch;  A,  pulmonary  artery;  i,  ductus  Botalli  of  the 
latter.  0.  it,  origin  of  aorta;  e,  fourth  arch  of  the  right  side  (root  of  dorsal  aorta); 
/,  right  subclavian;  g,  dorsal  aorta;  h,  left  subclavian  (fourth  arch  of  the  left 
side);  i,  pulmonary  artery;  k  and  I,  right  and  left  ductus  Botalli  of  the  pulmonary 
arteries.  D.  d,  origin  of  aorto;  e,  fourth  arch  of  the  left  side  (root  of  dorsal 
aorta);  f,  dorsal  aorta;  q,  left  vertebral  artery;  h,  left  subclavian;  i,  right  sub- 
clavian'(fourth  arch  of  the  right  side);  k.  right  vertebral  artery;  I,  continuation  of 
the  right  subclavian;  in.  pulmonary  artery;  n,  ductus  Botalli  of  the  latter  (usually 
termed  ductus  arteriosus). 

With  the  development  of  the  placenta  the  allantoic  circula- 
tion renders  the  vitelline  subordinate,  the  vitelline  and  the  larger 
mesenteric  vein  forming  the  portal.  The  portal  vein  at  a  later 
period  joins  one  of  the  vena}  advehentes  of  the  allantoic  vein. 

At  first  the  vena  cava  inferior  and  the  ductus  venosus  enter 
the  heart  as  a  common  trunk.  The  ductus  venosus  Arantii  be- 
comes a  small  branch  of  the  vena  cava. 


THE   DEVELOPMENT  OF  THE  EMBRYO  ITSELF.   \\\ 


The  allantoic  vein  is  finally  represented  in  its  degenerated 
form  as  a  solid  cord  {round  ligament),  the  entire  venous  sup- 
ply of  the  liver  being  derived  from  the  portal  vein. 

The  development  of  the  heart  has  already  been  traced  in  the 
fowl  up  to  a  certain  point.  In  the  mammal  its  origin  and  early 
progress  are  similar  and  its  further  history  may  be  gathered 
from  the  following  series  of  representations. 

In  the  fowl  the  heart  shows  the  commencement  of  a  division 
into  a  right  and  left  half  on  the  third  day,  and  about  the  fourth 
week  in  man,  from  which  fact  alone  some  idea  may  be  gained 
as  to  the  relative  rate  of  development.  The  division  is  effected 
by  the  outgrowth  of  a  septum  from  the  ventral  wall,  which  rap- 


Fig.  118. 


Fig.  117. 


?!'•— Development  of  the  heart  in  the  human  embryo,  from  the  fourth  to  the 
sixth  week.  A.  hmbryo  of  four  weeks  (KOlliker,  after  Coste).  B,  anterior  C 
posterior  views  of  the  heart  of  an  embryo  of  six  weeks  (KOlliker.  after  Eck'er)' 
a  upper  limit  of  buccal  cavity;  c,  buccal  cavity;  b,  lies  between  the  ventral  ends 
of  second  and  third  branchial  arches;  d,  buds  of  upper  limbs;  e  liver-  f  intes- 
tine; 1,  superior  vena  cava:  1',  left  superior  vena  cava;  1",  opening  of  inferior 
bu?b  CaVa'  aM  'eft  anricles;  3-  3'-  ri§ht  and  left  ventricles;  4,  aortic 

Fig.  118.— Human  embryo  of  about  three  weeks  (Allen  Thomson),    uv.  yelk-sac-  al 
allantois;  am,  amnion;  ae,  anterior  extremity;  pe.  posterior  extremity. 

idly  reaches  the  dorsal  side,  when  the  double  ventricle  thus 
formed  communicate  by  a  right  and  a  left  auriculo-ventricular 
opening  with  the  large  and  as  yet  undivided  auricle.  Later  an 
incomplete  septum  forms  similar  divisions  in  the  auricle  ;  the 
aperture  {foramen  ovale)  left  by  the  imperfect  growth  of  this 
wall  persisting  throughout  foetal  life. 

The  Eustachian  valve  arises  on  the  dorsal  wall  of  the  right 
auricle,  between  the  vena  cava  inferior  and  the  right  and  left 


112 


COMPARATIVE   PHYSIOLOGY. 


venae  cavaa  superiores ;  but  in  many  mammals,  among  which  is 
man,  the  left  vena  cava  superior  disappears  during  fcetal  life. 

For  the  present  we  may  simply  say  that  the  histories  of  the 
development  of  the  heart,  the  blood-vessels,  and  the  blood  itself 
are  closely  related  to  each  other,  and  to  the  nature  and  changes 
of  the  various  methods  in  which  oxygen  is  supplied  to  the  blood 
and  tissues,  or,  in  other  words,  to  the  development  of  the  respir- 
atory system. 

THE   DEVELOPMENT  OF   THE   UROGENITAL   SYSTEM. 

Without  knowing  the  history  of  the  organs,  the  anatomical 
relations  of  parts  with  uses  so  unlike  as  reproduction  on  the  one 
hand  and  excretion  on  the  other,  can  not  be  comprehended;  nor, 
as  will  be  shortly  made  clear,  the  fact  that  the  same  part  may 
serve  at  one  time  to  remove  waste  matters  (urine)  and  at  an- 
other the  generative  elements. 

The  vertebrate  excretory  system  may  be  divided  into  three 
parts,  which  result  from  the  differentiation  of  the  primitive  kid- 
ney which  has  been  effected  during  the  slow  and  gradual  evo- 
lution of  vertebrate  forms : 

1.  The  head-kidney  (pronephros). 

2.  The  Wolffian  body  (mesonephros). 

3.  The  kidney  proper,  or  metanephros. 

But  in  this  instance,  as  in  others  to  some  of  which  allusion 
has  already  been  made,  these  three  parts  are  not  functional  at 
the  same  time.  The  pronephros  arises  from  the  anterior  part 
of  the  segmental  duct,  pronephric  duct,  duct  of  primitive  kid- 
ney, and  archinephric  duct,  and  in  the  fowl  is  apparent  on  the 
third  day ;  but  the  pronephros  is  best  developed  in  the  ichthy- 


Pio.  110. — Diagrams  illustrating  development  of  pronephron  in  the  fowl  (Ilnddon). 
ao,  aorta;  o.c,  body-cavity;  ep,  epiblast  with  its  epitrichial  (flattened)  layer;  hy, 
hypoblast;  m.  s,  mesoblastic  somite;  n.  c,  neural  canal;  nch,  notochord;  p.  I..,  pro- 
nephric tubule;  so,  somatic,  and  up,  splanchnic  mesoblaBt. 


THE   DEVELOPMENT  OF  THE  EMBRYO  ITSELF.    H3 

opsida  (fishes  and  amphibians).  A  vascular  process  from  the 
peritoneum  (glomerulus)  projects  into  a  dilated  section  of  the 
body  cavity,  which  is  in  part  separated  from  the  rest  of  this 
cavity  (ccelom).  This  process,  together  with  the  segmental  duct, 
now  coiled,  and  certain  short  tubes  developed  from  the  original 
duct,  make  up  the  pronephros.  The  segmental  duct  opens  at 
length  into  the  cloaca. 

The  mesonephros  (Wolffian  body),  though  largely  developed 
in  all  vertebrates  during  fcetal  life,  is  not  a  persistent  excretory 
organ  of  adult  life. 

In  the  fowl  recent  investigation  has  shown  that  the  Wolffian 
(segmental)  tubes  originate  from  outgrowths  of  the  Wolffian 


Fig.  120. 


Fig.  181. 


Fig.  120. — Rudimentary  primitive  kidney  of  embryonic  dog.  The  posterior  portion  of 
the  body  of  the  embryo  is  seen  from  the  ventral  side,  covered  by  the  intestinal 
layer  of  the  yelk-sac.  which  has  been  torn  away,  and  thrown  back  in  front  in 
order  to  show  the  primitive  kidney  ducts  with  the  primitive  kidney  tubes  (a),  h, 
primitive  vertebrae;  c,  dorsal  medulla;  d,  passage  into  the  pelvic  intestinal  cavity. 
(Hacckel.  alter  Bischoff.) 

Fig.  121. — Primitive  kidney  of  a  human  embryo,  v.  the  urine-tubes  of  the  primitive 
kidney:  ir.  Wolffian  duct;  w',  upper  end  of  the  latter  (Morgagni's  hydatid*:  m, 
Mfillerian  duct;  m'.  upper  end  of  the  latter  (Fallopian  hydatid):  g,  hermaphrodite 
>    gland.    (After  Kobelt.) 

duct  aud  also  from  an  intermediate  cell-mass,  from  which  latter 
the  Malpighian  bodies  take  rise.     The(  tubes,  at  first  not  con- 


114 


COMPARATIVE   PHYSIOLOGY. 


nected  with  the  duct,  finally  join  it.  This  organ  is  continuous 
with  the  pronephros  ;  in  fact,  all  three  (pronephros,  rnesone- 
phros,  and  metanephros)  may  be  regarded  as  largely  continua- 
tions one  of  another. 

The  metanephros,  or  kidney  proper,  arises  from  mesoblast  at 
the  posterior  part  of  the  Wolffian  body.     The  ureter  originates 


Fig.  122.— Section  of  the  intermediate  ceil-mass  of  fourth  day  (Foster  and  Balfour, 
after  Waldeyer).  1  x  160.  m.  mesentery;  L.  somatopleure;  «',  portion  of  the 
germinal  epithelium  from  the  duct  of  Miiller  is  formed  by  involution;  a,  thick- 
ened portion  of  the  germinal  epithelium,  in  which  the  primitive  ova,  C  and  o.  are 
lying;  E,  modified  mesoblast  which  will  form  the  stroma  of  the  ovary;  WK, 
Wolffian  body;  y,  Wolffian  duct. 

first  from  the  hinder  portion  of  the  Wolffian  duct.  In  the  fowl 
the  kidney  tubules  bud  out  from  the  m^eter  as  rounded  eleva- 
tions. The  ureter  loses  its  connection  wTith  the  Wolffian  duct 
and  opens  independently  into  the  cloaca. 

The  following  account  will  apply  especially  to  the  higher 
vertebrates  : 

The  segmental  (archinephric)  duct  is  divided  horizontally 
into  a  dorsal  or  Wolffian  (mesonephric)  duct  and  a  ventral  or 
Miillerian  duct.  The  Wolffian  duct,  as  we  have  seen,  develops 
into  both  ureter  and  kidney  proper. 

To  carry  the  subject;  somewhat  further  back,  the  epithelium 


THE  DEVELOPMENT   OF    THE  EMBRYO   ITSELF.    115 

lining  the  coelom  at  one  region  becomes  differentiated  into  col- 
umnar cells  {germinal  epithelium)  which  by  involution  into 
the  underlying  mesoblast  forms  a  tubule  extending  from  before 
backward  and  in  close  relation  with  the  Wolffian  duct,  thus 
forming  the  Mullerian  duct  by  the  process  of  cleavage  and 
separation  referred  to  previously. 


Fig.  123. — Diagrammatic  representation  of  the  genital  organs  of  a  human  embryo  pre- 
vious to  sexual  distinction  (Allen  Thomson!.  TT".  Wolffian  body;  gc,  genital  cord; 
m,  Mlillerian  duct:  w,  Wolffian  duct:  ug,  urogenital  sinus;  cp.  clitoris  or  penis; 
i,  intestine;  cl,  cloaca;  Is,  part  from  which  the  scrotum  or  labia  majora  are  devel- 
oped; ot,  origin  of  the  ovary  or  testicle  respectively;  x.  part  of  the  Wolffian  body 
developed  later  into  the  cohi  vasculosis  3,  ureter:  4.  bladder:  5,  urachus. 


The  future  of  the  Mullerian  and  Wolffian  ducts  varies  ac- 
cording to  the  sex  of  the  embryo. 

In  the  male  the  Wolffian  duct  persists  as  the  vas  deferens  ; 
in  the  female  it  remains  as  a  rudiment  in  the  region  near  the 
ovary  (hydatid  of  Morgagni).  In  the  female  the  Mulleriau  duct 
becomes  the  oviduct  and  related  parts  (uterus  and  vagina)  ;  in 
the  male  it  atrophies.  One,  usually  the  right,  also  atrophies  in 
female  birds.  The  sinus  pocularis  of  the  pi'ostate  is  the  remnant 
in  the  male  of  the  fused  tubes. 

The  various  forms  of  the  generative  apparatus  derived  from 
the  Mullerian  ducts,  as  determined  by  different  degrees  of  fu- 


116 


COMPARATIVE  PHYSIOLOGY. 


Fig.  134. — Diagram  of  the  mammalian  type  of  male  sexual  organs  (after  Quain).  Com- 
pare with  Figs.  123,  125.  C.  Cowper's  gland  of  one  side;  cp,  corpora  cavernosa 
penis,  cut  short;  e,  caput  epididymis;  g,  gubernaculum;  i,  rectum;  m,  hydatid  of 
Morgagni,  the  persistent  anterior  end  of  the  Miillerian  duct,  the  conjoint  posterior 
ends  of  which  form  the  uterus  masculinus;  pi',  prostate  gland;  s,  scrotum;  sp, 
corpus  spongiosum  urethra;  t,  testis  (testicle)  in  the  place  of  its  original  forma- 
tion. The  dotted  line  indicates  the  direction  in  which  the  testis  and  epididymis 
change  place  in  their  descent  from  the  abdomen  into  the  scrotum;  vd,  vas  defer- 
ens; vh,  vas  aberrans;  vs,  vasicula  seminalis;  W,  remnants  of  Wolffian  body  (the 
organ  of  Giraldds  or  paradidymis  of  Waldeyer);  3,  4,  5,  as  in  Fig.  125. 


sion,  etc.,  of  parts,  may  be  learned  from  the  accompanying 
figures. 


n    v    C 


Fio.  125.— Diagram  of  the  mammalian  type  of  female  sexual  organs  (after  Quain). 
The  dotted  iines  in  one  figure  indicate  functional  organs  in  the  other.  C,  gland  of 
Bartholin  (OowperV  gland);  <■./:,  corpus  cavernosum  clitoridis;  dQ,  remains  of  the 
left  Wolffian  duct,  which  may  persist  as  the  duct  of  Gaertner;  f,  abdominal  open- 
ing of  left  Fallopian  tube;  ground  ligament  (corresponding  tofhegubernaculum); 
//.hymen;  i,  rectum;  I,  labium;  m,  cut  Fallopian  tube  (oviduct,  or  Miillerian  duct) 
of  the  right  wide;  //,  nyinpha;  o,  left  ovary;  po.  parovarium;  SO,  vascular  bulb  or 
corpus  spongiosum;  u,  uterus;  v,  vulva;  va,  vagina;  W,  scattered  remains  of  Wol- 
ffian tubes  (paroophoron);  w,  cut  end  of  vanished  right  Wolffian  duct;  3,  ureter;  4, 
•bladder  passing  below  into  tin.' urethra  ;  5,  urachus,  or  remnant,  of  stalk  of  allantois. 


THE  DEVELOPMENT   OF   THE  EMBRYO   ITSELF.    Hf 

In  both  sexes  the  most  posterior  portion  of  the  Wolffian  duct 
gives  rise  to  the  metanephros,  or  what  becomes  the  permanent 
kidney  and  ureter  ;  in  the  male  also  to  the  vas  deferens,  testicle, 
vas  aberrans,  and  seminal  vesicle. 

The  ovary  has  a  similar  origin  to  the  testicle  ;  the  germinal 
epithelium  furnishing  the  cells,  which  are  transformed  into 
Graafian  follicles,  ova,  etc.,  and  the  mesoblast  the  stroma  in 
which  these  structures  are  imbedded. 

In  the  female  the  parovarium  remains  as  the  representative 
of  the  atrophied  Wolffian  body  and  duct. 

The  bladder  and  urachus  are  both  remnants  of  the  formerly 
extensive  allantois.  The  final  forms  of  the  genito-urinary  or- 
gans arise  by  differentiation,  fusion,  and  atrophy  :  thus,  the 
cloaca  or  common  cavity  of  the  genito-urinary  ducts  is  divided 


ALL, 


ALL 


Fig.  128.  Fig.  129. 

Figs.  126  to  129.— Diagrams  illustrating  the  evolution  of  the  posterior  passages  (after 

Landois  and  Stirling). 
Fig.  126. — Allantois  continuous  with  rectum. 
Pig.  127.— Cloaca  formed. 
Fig.  128. — Early  condition  in  male,  before  the  closure  of  the  folds  of  the  groove  on 

the  posterior  side  of  the  penis. 
Fig.  129. — Early  female  condition. 
A,  commencement  of  proctodeum;  ALL,  allantois;  B.  bladder;  C.  penis;  CL,  cloaca; 

M,  Miillerian  duct;  J?,  rectum;  U,  urethra;  S,  vestibule;  SU,  urogenital  sinus:   1'. 

vas  deferens  in  Fig.  128,  vagina  in  Fig.  129. 

by  a  septum  (the  perineum  externally)  into  a  genito-urinary 
and  an  intestinal  (anal)  part  ;  the  penis  in  the  male  and  the 
corresponding  clitoris  in  the  female  appear  in  the  region  of  the 
cloaca,  as  outgrowths  which  are  followed  by  extension  of  folds 
of  integument  that  become  the  scrotum  in  the  one  sex  and  the 
labia  in  the  other. 

The  urethra  arises  as  a  groove  in  the  under  surface  of  the 


118 


COMPARATIVE   PHYSIOLOGY. 


penis,  which  becomes  a  canal.  The  original  opening1  of  the 
urethra  was  at  the  base  of  the  penis. 

In  certain  cases  development  of  these  parts  is  arrested  at 
various  stages,  from  which  result  abnormalities  frequently  re- 
quiring interference  by  the  surgeon. 

The  accounts  of  the  previous  chapters  do  not  complete  the 
history  of  development.  Certain  of  the  remaining  subjects 
that  are  of  special  interest,  from  a  physiological  point  of  view, 
will  be  referred  to  again  ;  and  in  the  mean  time  we  shall  con- 
sider rather  briefly  some  of  the  physiological  problems  of  this 
subject  to  which  scant  reference  has  as  yet  been  made.  Though 
the  physiology  of  reproduction  is  introduced  here,  so  that  ties 
of  natural  connection  may  not  be  severed,  it  may  very  well  be 
omitted  by  the  student  who  is  dealing  with  embryology  for  the 


Pig.  130.— Various  forms  of  mammalian  uteri.  A.  Ornithorhynchus.  B.  Didelphys 
dorsigera.  C.  Phalangista  vulpina.  D.  Double  uterus  and  vagina;  human  anoma- 
ly. E.  Lepus  cuniculus  (rabbit),  uterus  duplex.  F.  Uterus  bicornis.  G.  Uterus 
bipartitus.  H.  Uterus  simplex  (human),  a,  anus;  cl,  cloaca;  o.  d,  oviduct;  o.  I, 
os  tincse  (os  uteri);  ov,  ovary:  r,  rectum;  s,  vaginal  septum;  u.  b,  urinary  bladder; 
ur,  ureter;  ur.  o,  orifice  of  same;  u.  s,  urogenital  sinus;  ut,  uterus;  v,  vagina;  v.  c, 
vaginal  caecum  (Haddon). 

first  time,  and  in  any  case  should  be  read  again  after  the  other 
functions  of  the  body  have  been  studied. 


THE   PHYSIOLOGICAL   ASPECTS    OF   DEVELOPMENT. 

According  to  that  law  of  rhythm  which,  as  we  have  seen, 
prevails  throughout  the  world  of  animated  nature,  there  are 
periods  of  growth  and  progress,  of  quietude  and  arrest  of  devel- 


THE  DEVELOPMENT   OF  THE  EMBRYO   ITSELF.    H9 

opment  ;  and  in  vertebrates  one  of  the  most  pronounced  epochs 
—  in  fact,  the  most  marked  of  all — is  that  by  which  the  young 
organism,  through  a  series  of  rapid  stages,  attains  to  sexual  ma- 
turity. 

While  the  growth  and  development  of  the  generative  organs 
share  to  the  greatest  degree  in  this  progress,  other  parts  of  the 
body  and  the  entire  being  participate. 

So  great  is  the  change  that  it  is  common  to  indicate,  in  the 
case  of  the  human  subject,  the  developed  organism  by  a  new 
name — the  "  boy  "  becomes  the  "  man,"  the  "  girl "  the  "  woman.1' 
Relatively  this  is  by  far  the  most  rapid  and  general  of  all  the 
transformations  the  organism  undergoes  during  its  extra-uter- 
ine life.  In  this  the  entire  body  takes  part,  but  very  unequally. 
The  increase  in  stature  is  not  proportionate  to  the  increase  in 
weight,  and  the  latter  is  not  so  great  as  the  change  in  form. 
The  modifications  of  the  organism  are  localized  and  yet  affect 
the  whole  being.  The  outlines  become  more  rounded  ;  the  pel- 
vis in  females  alters  in  shape ;  not  only  do  the  generative  organs 
themselves  rapidly  undergo  increased  development,  but  certain 
related  glands  (mammae)  participate  ;  hair  appears  in  certain 
regions  of  the  body  ;  the  larynx,  especially  in  the  male,  under- 
goes enlargement  and  changes  in  the  relative  size  of  parts,  re- 
sulting in  an  alteration  of  voice  (breaking  of  the  voice),  etc. — 
all  in  conformity  with  that  excess  of  nutritive  energy  which 
marks  this  biological  epoch. 

Correlated  with  these  physical  changes  are  others  belonging 
to  the  intellectual  and  moral  (psychic)  nature  equally  impor- 
tant, and,  accordingly,  the  future  being  depends  largely  on  the 
full  and  un warped  developments  of  these  few  years. 

Sexual  maturity,  or  the  capacity  to  furnish  ripe  sexual  ele- 
ments (cells),  is  from  the  biological  standpoint  the  most  impor- 
tant result  of  the  onset  of  that  period  termed,  as  regards  the 
human  species,  puberty. 

The  age  at  which  this  epoch  is  reached  varies  with  race, 
sex,  climate,  and  the  moral  influences  which  envelop  the  indi- 
vidual. In  temperate  regions  and  with  European  races  puberty 
is  reached  at  from  about  the  thirteenth  to  the  eighteenth  year 
in  the  female,  and  rather  later  in  the  male,  in  whom  develop- 
ment generally  is  somewhat  slower.  Changes  analogous  to  the 
above  occur  in  all  vertebrates.  It  is  at  this  period  that  differ- 
ences of  form,  voice,  disposition,  and  other  physical  and  psychic 
characteristics  first  become  pronounced. 


120 


COMPARATIVE   PHYSIOLOGY. 


As  a  matter  of  fact,  the  pig,  sheep,  goat,  cat,  dog,  and  certain 
other  animals  may  conceive  when  less  than  one  year  old ;  and  the 
cow  and  the  mare  when  under  two  years. 

At  such  periods  these  animals  are  not  of  course  mature  and 
should  not  be  bred. 

OVULATION. 

In  all  vertebrates,  at  periods  recurring  with  great  regularity, 
the  generative  organs  of  the  female  manifest  unusual  activity. 
This  is  characterized  by  increased  vascularity  of  the  ovary  and 
adjacent  parts  ;  with  other  changes  dependent  on  this,  and  that 
heightened  nerve  influence  which,  in  the  vertebrate,  seems  to  be 
inseparable  from  all  important  functional  changes.  Ovulation 
is  the  maturation  and  discharge  of  ova  from  the  Graafian  folli- 
cles. The  latter,  reaching  the  exterior  zone  of  the  ovaxy,  be- 
coming distended  and  thinned,  burst  externally  and  thus  free 
the  ovum.  The  follicles  being  very  vascular  at  this  period, 
blood  escapes,  owing  to  this  rupture,  into  the  emptied  capsule 
and  clots  ;  and  as  a  result  of  organization  and 
subsequent  degeneration  undergoes  a  certain 
series  of  changes  dependent  on  the  condition 
of  the  ovary  and  related  organs,  which  varies 
according  as  the  ovum  has  been  fertilized  or 
not.  When  fertilization  occurs  the  Graafian 
follicle  undergoes  changes  of  a  more  marked 
and  lasting  character,  becoming  a  true  corpus 
luteum  of  pregnancy. 

The  number  of  Graafian  follicles  that  ma- 
ture and  the  number  of  ripe  ova  that  escape  at 
about  the  same  period  varies,  of  course,  with 
the  species  and  the  individual,  and  is  not  al- 
ways the  same  in  the  latter. 

In  species  that  usually  bear  several  young 
at  a  birth  a  corresponding  number  of  ova  must 
be  ripened  and  fertilized  at  about  the  same 
time  ;  while  the  reverse  holds  for  those  that 
usually  give  birth  to  but  one. 

The  ovum  in  the  fowl  is  fertilized  in   the 
upper  part  of  the  oviduct;   in  the  mammal 
mostly  in  this  region  also,  as  is  shown  by  the 
site  of  the  embryos  in  those  groups  of  animals  with  a  two- 
horned  uterus,  and  the  occasional  occurrence  of  tubal  pregnan- 


Fio.  131.— Ovary  of 
rabbit  at  period 
of  oestrum,  show- 
ing various  stages 
of  extniHion  of 
ova.    (C'hauveau.) 


THE    DEVELOPMENT  OF  THE   EMBRYO   ITSELF.     121 

cy  in  woman.  But  this  is  not,  in  the  human  subject  at  least, 
invariably"  the  site  of  impregnation.  After  the  ovum  has  been 
set  free,  as  above  described,  it  is  conveyed  into  the  oviduct 
(Fallopian  tube),  though  exactly  how  is  still  a  matter  of  dis- 
pute :  some  holding  that  the  current  produced  by  the  action 
of  the  ciliated  cells  of  the  Fallopian  tube  suffices ;  others  that 
the  ovum  is  grasped  by  the  fimbriated  extremity  of  the  tube  as 
part  of  a  co-ordinated  act.  It  is  likely,  as  in  so  many  other 
instances,  that  both  views  are  correct  but  partial;  that  is  to 
say,  both  these  methods  are  employed.  The  columnar  ciliated 
cells,  lining  the  oviduct,  act  so  as  to  produce  a  current  in  the 
direction  of  the  uterus,  thus  assisting  the  ovum  in  its  passage 
toward  its  final  resting  place. 

CEstrum. — As  a  part  of  the  general  activity  occurring  at  this 
time,  the  uterus  manifests  certain  changes,  chiefly  in  its  inter- 
nal mucous  lining,  in  which  thickening  and  increased  vascular- 


Fio.  132.— Diagram  of  the  human  uterus 
just  before  menstruation.  The  shaded 
portion  represents  the  mucous  mem- 
brane (Hart  and  Barbour,  after  J. 
Williams). 


Fie  133.— Uterus  after  menstruation  has 
just  ceased.  The  cavity  of  the  body 
of  the  uterus  is  supposed  to  have 
been  deprived  of  mucous  membrane 
(J.  Williams). 


ity  are  prominent.  A  flow  of  blood  from  the  uterus  in  the  form 
of  a  gentle  oozing  follows;  and  as  the  superficial  parts  of  the 
mucous  lining  of  the  uterus  undergo- softening  and  fatty  degen- 


122  COMPARATIVE  PHYSIOLOGY. 

eration,  they  are  thrown  off  and  renewed  at  these  periods  (cata- 
vienia,  menses,  etc.),  provided  pregnancy  does  not  take  place. 
In  mammals  helow  man,  in  their  natural  state,  pregnancy  does 
almost  invariably  take  place  at  such  times,  hence  this  exalted 
activity  of  the  mucous  coat  of  the  uterus,  in  preparation  for  the 
reception  and  nutrition  of  the  ovum,  is  not  often  in  vain.  In 
the  human  subject  the  menses  appear  monthly ;  pregnancy  may 
or  may  not  occur,  and  consequently  there  may  be  waste  of  na- 
ture's forces ;  though  there  is  a  certain  amount  of  evidence  that 
menstruation  does  not  wholly  represent  a  loss;  but  that  it  is 
largely  of  that  character  among  a  certain  class  of  women  is 
only  too  evident.  As  can  be  readily  understood,  the  catamenial 
flow  may  take  place  prior  to,  during,  or  after  the  rupture  of  the 
egg-capsule. 

As  the  uterus  is  well  supplied  with  glands,  during  this 
period  of  increased  functional  activity  of  its  lining  membrane, 
mucus  in  considerable  excess  over  the  usual  quantity  is  dis- 
charged ;  and  this  phase  of  activity  is  continued  for  a  time  should 
pregnancy  occur. 

All  the  parts  of  the  generative  organs  are  supplied  with 
muscular  tissue,  and  with  nerves  as  well  as  blood-vessels,  so 
that  it  is  possible  to  understand  how,  by  the  influence  of  nerve- 
centers,  the  various  events  of  ovulation,  menstruation,  and 
those  that  follow  when  pregnancy  takes  place,  form  a  related 
series,  very  regular  in  their  succession,  though  little  prominent 
in  the  consciousness  of  the  individual  animal  when  normal. 

In  all  animals,  without  exception,  the  disturbance  of  the 
generative  organs  during  the  rutting  season  (oestrum)  is  accom- 
panied by  unusual  excitement  and  special  alterations  in  the 
temper  and  disposition,  while  it  may  perhaps  be  said  that  the 
whole  organism  is  correspondingly  affected. 

The  frequency  of  the  season  of  heat  or  rutting  is  variable,  as 
also  its  duration.  In  most  of  the  domestic  animals  it  lasts  but  a 
few  days;  though  in  the  bitch  it  may  be  prolonged  for  a 
month. 

It  is  not  common  for  conception  to  occur  in  the  human  sub- 
ject while  the  young  one  is  being  suckled,  and  the  same  remark 
applies  to  the  domestic  animals,  though  less  so,  and  with  con- 
siderable variation  for  different  species. 

Naturally,  the  periods  of  oestrum  will  depend  considerably 
on  the  occurrence  of  impregnation  and  the  duration  of  gesta- 
tion.    It  is  usual  for  the  mare  to  be  in  season  in  spring  and  fall, 


THE   DEVELOPMENT   OP  THE  EMBRYO   ITSELF.    123 

but,  of  course,  if  impregnated  in  the  spring,  there  will  be  no  au- 
tumn oestrum  on  account  of  the  prolonged  period  of  gestation 
in  this  instance ;  and,  similarly,  in  the  case  of  the  cow  and  other 
animals. 

It  is  important  to  recognize  that  rutting  is  only  the  evidence 
of  the  maturation  of  the  Graafian  follicle  within  the  ovary  and 
of  correlated  changes. 

In  a  state  of  nature — i.  e.,  in  the  case  of  wild  animals — the 
male  experiences  a  period  of  sexual  excitement  corresponding 
with  an  increased  activity  of  the  sexual  organs  and  at  periods 
answering  to  the  rutting  season  of  the  female.  In  some  species 
the  testes  descend  into  the  scrotum  only  at  this  season.  This 
may  be  observed  in  the  rabbit.  But  in  our  domestic  animals,  as 
a  class,  the  male,  though  capable  of  copulation  at  all  times,  is  ex- 
cited only  by  the  presence  of  a  female  in  season.  It  is  only  at 
such  periods  that  the  approach  of  the  male  is  permitted  by  the 
opposite  sex. 

THE   NUTRITION   OF   THE   OVUM   (OOSPERM). 

This  will  be  best  understood  if  it  be  remembered  that  the 
ovum  is  a  cell,  undifferentiated  in  most  directions,  and  thus  a 
sort  of  amoeboid  organism.  In  the  fowl  it  is  known  that  the 
cells  of  the  primitive  germ  devour,  amoeba-like,  the  yelk-cells, 
while  in  the  mammalian  oviduct  the  ovum  is  surrounded  by 
abundance  of  proteid,  which  is  doubtless  utilized  in  a  somewhat 
similar  fashion,  as  also  in  the  uterus  itself,  until  the  embryonic 
membranes  have  formed.  To  speak  of  the  ovum  being  nour- 
ished by  diffusion,  and  especially  by  osmosis,  is  an  unnecessary 
assumption,  and,  as  we  believe,  at  variance  with  fundamental 
principles;  for  we  doubt  much  whether  any  vital  process  is 
one  of  pure  osmosis.  As  soon  as  the  yelk-sac  and  allantois 
have  been  formed,  nutriment  is  derived  in  great  part  through 
the  vessel-walls,  which,  it  will  be  remembered,  are  differenti- 
ated from  the  cells  of  the  mesoblast,  and,  it  may  well  be  as- 
sumed, have  not  at  this  early  stage  entirely  lost  their  amoeboid 
character.  The  blood-vessels  certainly  have  a  respiratory  func- 
tion, and  suffice  till  the  more  complicated  villi  are  formed. 
The  latter  are  in  the  main  similar  in  structure  to  the  villi  of  the 
alimentary  tract,  and  are  adapted  to  being  surrounded  by  sim- 
ilar structures  of  maternal  origin.  Both  the  maternal  crypts 
and  the  foetal  villi  are,  though  complementary  in  shape,  all  but 


124  COMPARATIVE  PHYSIOLOGY. 

identical  in  minute  structure  in  most  instances.  In  each  case 
the  blood-vessels  are  covered  superficially  by  cells  which  we 
can  not  help  thinking  are  essential  in  nutrition.  The  villi  are 
both  nutritive  and  respiratory.  It  is  no  more  difficult  to  under- 
stand their  function  than  that  of  the  cells  of  the  endoderm  of  a 
polyp,  or  the  epithelial  coverings  of  lungs  or  gills. 

Experiment  proves  that  there  is  a  respiratory  interchange 
of  gases  between  the  maternal  and  fcetal  blood  which  nowhere 
mingle  physically.  The  same  law  holds  in  the  respiration  of 
the  foetus  as  in  the  mammals.  Oxygen  passes  to  the  region 
where  there  is  least  of  it,  and  likewise  carbonic  anhydride.  If 
the  mother  be  asphyxiated  so  is  the  foetus,  and  indeed  more 
rapidly  than  if  its  own  umbilical  vessels  be  tied,  for  the  mater- 
nal blood  in  the  first  instance  abstracts  the  oxygen  from  that 
of  the  foetus  when  the  tension  of  this  gas  becomes  lower  in  the 
maternal  than  in  the  fcetal  blood;  the  usual  course  of  affairs 
is  reversed,  and  the  mother  satisfies  the  oxygen  hunger  of  her 
own  blood  and  tissues  by  withdrawing  that  which  she  recently 
supplied  to  the  foetus.  It  will  be  seen,  then,  that  the  embryo  is 
from  the  first  a  parasite.  This  explains  that  exhaustion  which 
pregnancy,  and  especially  a  series  of  gestations,  entails.  True, 
nature  usually  for  the  time  meets  the  demand  by  an  excess  of 
nutritive  energy :  hence  many  animals  are  never  so  vigorous  in 
appearance  as  when  in  this  condition ;  often,  however,  to  be  fol- 
lowed by  corresponding  emaciation  and  senescence.  The  full 
and  frequent  respirations,  the  bounding  pulse,  are  succeeded  by 
reverse  conditions  ;  action  and  reaction  are  alike  present  in  the 
animate  and  inanimate  worlds.  Moreover,  it  falls  to  the  parent 
to  eliminate  not  only  the  waste  of  its  own  organism  but  that  of 
the  foetus ;  and  not  infrequently  in  the  human  subject  the  over- 
wrought excretory  organs,  especially  the  kidneys,  fail,  entailing 
disastrous  consequences. 

The  digestive  functions  of  the  embryo  are  naturally  inact- 
ive, the  blood  being  supplied  with  all  its  needful  constituents 
through  the  placenta  by  a  much  shorter  process  ;  indeed,  the 
placental  nutritive  functions,  so  far  as  the  foetus  is  concerned, 
may  be  compared  with  the  removal  of  already  digested  ma- 
terial from  the  alimentary  canal,  though  of  course  only  in  a 
general  way.  During  fcetal  life  the  digestive  glands  are 
developing,  and  at  the  time  of  birth  all  the  digestive  juices 
are  secreted  in  an  efficient  condition,  though  only  relatively 
so,  necessitating  a  special  liquid  food  (milk)  in  a  form  in  which 


THE  DEVELOPMENT   OP  THE  EMBRYO   ITSELF.    125 

all  the  constituents  of  a  normal  diet  are  provided,  easy  of  diges- 
tion. 

Bile,  inspissated  and  mixed  with  the  dead  and  cast-off  epi- 
thelium of  the  alimentary  tract,  is  abundant  in  the  intestine  at 
birth  ;  but  bile  is  to  be  regarded  perhaps  rather  in  the  light  of 
an  excretion  than  as  a  digestive  fluid.  The  skin  and  kidneys, 
though  not  functionless,  are  rendered  unnecessary  in  great  part 
by  the  fact  that  waste  can  be  and  is  withdrawn  by  the  placenta, 
which  proves  to  be  a  nutritive,  respiratory,  and  excretory  organ ; 
it  is  in  itself  a  sort  of  abstract  and  brief  chronicle  of  the  whole 
physiological  story  of  foetal  life. 

All  the  foetal  organs,  especially  the  muscles,  abound  in  an 
animal  starch  (glycogen),  which  in  some  way,  not  well  under- 
stood, forms  a  reserve  fund  of  nutritive  energy  which  is  pretty 
well  used  up  in  the  earlier  months  of  pregnancy.  We  may  sup- 
pose that  the  amceboid  cells — all  the  undifferentiated  cells  of 
the  body — feed  on  it  in  primitive  fashion;  and  it  will  not  be 
forgotten  that  the  older  the  cells  become,  the  more  do  they  de- 
part from  the  simpler  habits  of  their  earlier,  cruder  existence ; 
hence  the  disappearance  of  this  substance  in  the  later  months  of 
foetal  life. 

In  one  respect  the  foetus  closely  resembles  the  adult :  it  draws 
the  pabulum  for  all  its  various  tissues  from  blood  which  it- 
self may,  perhaps,  be  regarded  as  the  first  completed  tissue.  We 
are,  accordingly,  led  to  inquire  how  this  river  of  life  is  distrib- 
uted ;  in  a  word,  into  the  nature  of  the  foetal  circulation. 

Foetal  Circulation. — The  blood  leaves  the  placenta  by  the 
umbilical  vein,  reaches  the  inferior  vena  cava,  either  directly 
(by  the  ductus  venosus),  or,  after  first  passing  to  the  liver  (by 
the  venae  advehentes,  and  returning  by  the  venae,  revehent.es), 
and  proceeds,  mingled  with  the  blood  returning  from  the  lower 
extremities,  to  the  right  auricle.  This  blood,  though  far  from 
being  as  arterial  in  character  as  the  blood  after  bii'th,  is  the  best 
that  reaches  the  heart  or  any  part  of  the  organism.  After  arriv- 
ing at  the  right  auricle.being  dammed  back  by  the  Eustachian 
valve,  it  avoids  the  right  ventricle,  and  shoots  on  into  the  left 
auricle,  passing  thence  into  the  left  ventricle,  from  which  it  is 
sent  into  the  aorta,  and  is  tben  carried  by  the  great  trunks  of  this 
arch  to  the  head  and  iipper  extremities.  The  blood  returning 
from  these  parts  passes  into  the  right  auricle,  then  to  the  corre- 
sponding ventricle,  and  thence  into  the  pulmonary  artery;  but, 
finding  the  branches  of  this  vessel  unopened,  it  takes  the  line  of 


Pulmonary  Art 

Foramen  Ovale 

Eustachian  Valve. 
Bight  Auric. -Vent.  Opening. fc\%l./i!.  /   *%. 


Bladder 


Pulmonary  Art. 
Left  Auricle. 
.Left  Auric.  ■  Vent. 
Opening, 


Hepatic  Vein. 

Branches  of  the 
Umbilical  Vein,  . 
to  the  Liver. 


Ductus  Venoms. 


Internal  Iliac  Arteries. 
Via.  13-1.— Diagram  of  the  foetal  circulation  (Flint). 


THE   DEVELOPMENT   OP  THE   EMBRYO   ITSELF.    127 

least  resistance  through  the  ductus  arteriosus  into  the  aortic 
arch  beyond  the  point  "where  its  great  branches  emerge.  It  will 
be  seen  that  the  blood  going  to  the  head  and  upper  parts  of  the 
body  is  greatly  more  valuable  as  nutritive  pabulum  than  the  rest, 
especially  in  the  quantity  of  oxygen  it  contains ;  that  the  blood 
of  the  foetus,  at  best,  is  relatively  ill-supplied  with  its  vital  essen- 
tial ;  and  as  a  result  we  find  the  upper  (anterior  in  quadrupeds) 
parts  of  the  foetus  best  developed,  and  a  decided  resemblance  be- 
tween the  mammalian  foetus  functionally  and  the  adult  forms  of 
reptiles  and  kindred  groups  of  lower  vertebrates.  But  this  con- 
dition is  well  enough  adapted  to  the  general  ends  to  be  attained 
at  this  period — the  nourishment  of  structures  on  the  way  to  a 
higher  path  of  progress. 

As  embryonic  maturity  is  being  reached,  preparation  is  made 
for  a  new  form  of  existence  ;  so  it  is  found  that  the  Eustachian 
valve  is  less  prominent  and  the  foramen  ovale  smaller. 

PERIODS   OF   GESTATION. 

As  a  rule,  the  shorter  the  period  of  gestation  the  more  nu- 
merous the  offspring  at  a  single  birth  and  the  greater  the  num- 
ber produced  within  the  lifetime  of  the  animal  relatively  to  its 
duration.  Thus,  on  account  of  the  number  of  young  produced 
by  the  rabbit  at  one  birth,  the  short  period  of  gestation,  and  the 
frequency  with  which  impregnation  occurs,  there  is  a  much 
larger  number  of  progeny,  short  as  is  the  animal's  life  usually, 
than  in  the  case  of  the  cow,  for  example,  that  may  bear  young 
for  a  much  longer  period. 

The  following  table  gives  approximately  the  duration  of  the 
period  of  gestation  of  some  of  our  domestic  animals  and  their 
wild  allies  : 

Guinea-pig  (cavy) 3  weeks. 

Rabbit,  squirrel,  rat 4       " 

Ferret 6       " 

Cat S       " 

Dog,  fox 9       " 

Lion 4  months. 

Sow 4 

Sheep,  goat 5        " 

Bear 7        " 

Reindeer 8        " 

Cow,  buffalo 10        " 


128  COMPARATIVE   PHYSIOLOGY. 

Mare,  ass,  zebra 11  months. 

Camel 12        " 

Giraffe 14 

Elephant 22  to  25  " 

The  period  of  gestation  in  the  human  subject  is  nine  months; 
in  the  monkeys  and  apes  somewhat  less.  The  incubation  pe- 
riod of  certain  of  our  domestic  birds  and  related  species  is  about 
as  follows  : 

Canary 11  days. 

Pigeon 18      " 

Hen 21      " 

Duck,  goose,  pea-hen 28      " 

Guinea-hen 25 

Turkey : 28      '• 

It  is  interesting  to  note  that  the  smaller  varieties  of  fowls 
hatch  out  sooner  than  the  larger  ;  and  that  the  period  of  incu- 
bation of  all  of  the  above  varies  with  the  weather,  the  steadi- 
ness of  the  incubating  bird,  the  date  on  which  the  eggs  selected 
were  laid,  etc.  With  very  recent  eggs,  an  attentive  sitter,  and 
warm  weather,  the  incubation  period  is  shortened. 

PARTURITION. 

All  the  efforts  that  have  hitherto  been  made  to  determine 
the  exact  cause  of  the  result  of  that  series  of  events  which  make 
up  parturition  have  failed.  This  has  probably  been  owing  to 
an  attempt  at  too  simple  a  solution.  The  foetus  lies  surrounded 
(protected)  by  fluid  contained  in  the  amniotic  sac.  For  its  ex- 
pulsion there  is  required,  on  the  one  hand,  a  dilatation  of  the 
uterine  opening  (os  uteri),  and,  on  the  other,  an  expulsive  force. 
The  latter  is  furnished  by  the  contractions  of  the  uterus  itself, 
aided  by  the  simultaneous  action  of  the  abdominal  muscles. 
Throughout  the  greater  part  of  gestation  the  uterus  experiences 
somewhat  rhythmical  contractions,  feeble  as  compared  with  the 
final  ones  which  lead  to  expulsion  of  the  foetus,  but  to  be  re- 
garded as  of  the  same  character.  With  the  growth  and  func- 
tional development  of  other  organs,  the  placenta  becomes  of 
less  consequence,  and  a  fatty  degeneration  sets  in,  most  marked 
at  the  periphery,  usually  where  it  is  thinnest  and  of  least  use. 
It  does  not  seem  rational  to  believe  that  the  onset  of  labor  is 
referable  to  any  one  cause,  as  has  been  so  often  taught ;  but 
rather  that  it  is  the  final  issue  to  a  series  of  processes  long  ex- 


THE   DEVELOPMENT   OP   THE   EMBRYO   ITSELF.    129 

isting  and  gradually,  though  at  last  rapidly,  reaching  that 
climax  which  seerus  like  a  vital  storm.  The  law  of  rhythm 
affects  the  nervous  system  as  others,  and  upon  this  depends 
the  direction  and  co-ordination  of  those  many  activities  which 
make  up  parturition.  We  have  seen  that. throughout  the  whole 
of  foetal  life  changes  in  one  part  are  accompanied  by  correspond- 
ing changes  in  others  ;  and  in  the  final  chapter  of  this  history 
it  is  not  to  be  expected  that  this  connection  should  be  severed, 
though  it  is  not  at  present  possible  to  give  the  evolution  of  this 
process  with  any  more  than  a  general  approach  to  probable 
correctness. 

CHANGES  IN   THE    CIRCULATION  AFTER   BIRTH. 

When  the  new-born  mammal  takes  the  first  breath,  effected 
by  the  harmonious  action  of  the  respiratory  muscles,  excited  to 
action  by  stimuli  reaching  them  from  the  nerve-center  (or 
centers)  which  preside  over  respiration,  owing  to  its  being 
roused  into  action  by  the  lack  of  its  accustomed  supply  of 
oxygen,  the  hitherto  solid  lungs  are  expanded  ;  the  pulmonary 
vessels  are  rendered  permeable,  hence  the  blood  now  takes  the 
path  of  least  resistance  along  them,  as  it  formerly  did  through 
the  ductus  arteriosus.  The  latter,  from  lack  of  use,  atrophies 
in  most  instances.  The  blood,  returning  to  the  left  auricle  of 
the  heart  from  the  lungs  in  increased  volume,  so  raises  the 
pressure  in  this  chamber  that  the  stream  that  formerly  flowed 
through  the  foramen  ovale  from  the  right  auricle  is  opposed 
by  a  force  equal  to  its  own,  if  not  greater,  and  hence  passes  by 
an  easier  route  into  the  right  ventricle.  The  fold  that  tends  to 
close  the  foramen  ovale  grows  gradually  over  the  latter,  so  that 
it  usually  ceases  to  exist  in  a  few  days  after  birth. 

At  birth,  ligature  of  the  umbilical  cord  cuts  off  the  placental 
circulation  ;  hence  the  ductus  venosus  atrophies  and  becomes  a 
mere  ligament. 

The  placenta,  being  now  a  foreign  body  in  the  uterus,  is  ex- 
pelled, and  this  organ,  by  the  contractions  of  its  walls,  closes 
the  ruptured  and  gaping  vessels,  thus  providing  against  haemor- 
rhage. 

COITUS. 

In  all  the  higher  vertebrates  congress  of  the  sexes  is  essential 
to  bring  the  male  sexual  product  into  contact  with  the  ovum 
9 


130 


COMPARATIVE   PHYSIOLOGY. 


Erection  of  the  penis  results  from  the  conveyance  of  an  ex- 
cess of  blood  to  the  organ,  owing-  to  dilatation  of  its  arteries, 
and  the  retention  of  this  blood  within  its  caverns. 


Fig.  135.— Section  of  erectile  tissue  (Cadiat).    a,  trabecule  of  connective  tissue,  with 
elastic  fibers,  and  bundles  of  plain  muscular  tissue  (c);  b.  venous  spaces  (Schafer). 

The  structure  of  the  penis  is  peculiar,  and,  for  the  details  of 
the  anatomy  of  both  the  male  and  female  generative  organs, 
the  student  is  referred  to  works  on  this  subject  ;  suffice  it  to 
say  that  it  consists  of  erectile  tissue,  the  chief  characteristic  of 
which  is  the  opening  of  the  capillaries  into  cavernous  venous 
spaces  (sinuses)  from  which  the  veinlets  arise  ;  with  such  an 
arrangement  the  circulation  must  be  very  slow — the  inflow 
being  greatly  in  excess  of  the  outflow — apart  altogether  from 
the  compressive  action  of  certain  muscles  connected  with  the 
organ.  The  manner  and  duration  of  the  act  of  copulation  in 
the  domestic  animals  varies  with  the  structure  of  the  penis,  the 
animal's  nervous  excitability,  etc.  In  the  stallion  the  act  is  of 
moderate  duration,  the  penis  long,  and  the  glans  penis  highly 
sensitive. 

In  the  bull  the  penis  is  of  a  different  shape.  During  erec- 
tion it  is  believed  that  the  S-shaped  curve  disappears,  and  that 
the  extremity  of  the  organ  enters  the  mouth  of  the  uterus  itself. 
( !<»l>ulation  is  of  very  brief  duration. 

In  the  dog  the  penis  is  of  very  peculiar  formation.     Its  an- 


THE   DEVELOPMENT   OF   THE   EMBRYO  ITSELF.    131 


terior  part  contains  a  bone,  while  there  are  two  erectile  portions 
independent  of  each  other.     During  copulation  the  largest  (pos- 


Fig.  136.— Section  of  parts  of  three  seminiferous  tubules  of  the  rat  (Schfifer).  a,  with 
the  spermatozoa  least  advanced  in  development;  b,  more  advanced;  c.  containing 
fully  developed  spermatozoa.  Between  the  tubules  are  seen  strands  of  interstitial 
cells,  with  blood-vessels  and  lymph-spaces. 

terior)  erectile  region  is  spasmodically  (reflexly)  grasped  by  the 
sphincter  cunni  of  the  female,  which  is  the  analogu  e  of  the 
bulbo-cavernosus,  ischio-cavernosus,  and  deep  transverse  mus- 
cle of  the  perinaeum,  so  that  the  penis  can  not  be  withdrawn 
till  the  erection  subsides,  an  advantage,  considering  that  the 
seminal  vesicles  are  wanting  in  the  dog,  as  well  as  Cowper's 
glands.  In  the  cat  tribe  there  is  also  an  incomplete  penial 
bone.  The  free  poi*tion  of  the  organ  is  provided  with  rigid 
papillae  capable  of  erection  during  copulation. 

As  previously  explained,  the  spermatozoa  originate  in  the 
seminal  tubes,  from  which  they  find  their  "way  to  the  seminal 
vesicles  or  receptacles  for  semen  till  required  to  be  discharged. 
The  spermatozoa  as  they  mature  are  forced  on  by  fresh  addi- 
tions from  behind  and  by  the  action  of  the  ciliated  cells  of  the 
epididymis,  together  with  the  wave-like  (peristaltic)  action  of 
the  vas  deferens.  Discharge  of  semen  during  coitus  is  effected 
by  more  vigorous  peristaltic  action  of  the  vas  deferens  and  the 
seminal  vesicles,  followed  by  a  similar  rhythmical  action  of  the 
bulbo-cavernosus  and  ischio-cavernosus  muscles,  by  which  the 
fluid  is  forcibly  ejaculated. 


132 


COMPARATIVE  PHYSIOLOGY. 


PlG.  137. — Generative  organs  of  the  mare,  isolated  and  partly  opened  (Chanvean).  1,  1. 
ovaries;  2,  Si,  Fallopian  tubes;  3,  pavilion  of  tube,  external  face;  4,  the  same,  in- 
ner face,  showing  opening  in  the  middle;  5,  ligament  of  ovary;  o,  intact  horn  of 
uterus;  ',',  a  born  thrown  open;  8,  body  of  uterus,  upper  face;  !),  broad  ligament; 
10,  cervix,  with  its  mucous  folds;  11,  cul-de-sac  of  vagina  with  its  folds  of  mucous 
membrane;  13,  urinary  meatus  and  its  valve,  14;  15,  mucous  fold,  a  vestige  of  hy- 
men; 10,  interior  of  vulva;  17,  clitoris;  18, 18,  labia  of  vulva;  19,  inferior  commis- 
sure of  vulva. 


THE   DEVELOPMENT   OF   THE   EMBRYO   ITSELF.    133 

Semen  itself,  though  composed  essentially  of  spermatozoa, 
is  mixed  with  the  secretions  of  the  vas  deferens,  of  the  seminal 
vesicles,  of  Cowper's  glands,  and  of  the  prostate.  Chemically  it 
is  neutral  or  alkaline  in  reaction,  highly  albuminous,  and  con- 
tains nuclein,  lecithin,  cholesterin,  fats,  and  salts. 

The  movements  of  the  male  cell,  owing  to  the  action  of  the 
tail  (cilium),  suffice  of  themselves  to  convey  them  to  the  ovi- 
ducts ;  but  there  is  little  doubt  that  during  or  after  sexual  con- 
gress there  is  in  the  female,  even  in  the  human  subject,  at  least 


Fig.  138  —Uterus  and  ovaries  of  the  sow.  semi-diagrammatic  (after  Dalton).    0,  ovary; 
II,  Fallopian  tube;  h,  horn  of  the  uterus;  5,  body  of  the  uterus;  v,  vagina. 

in  many  cases,  a  retrograde  peristalsis  of  the  uterus  and  ovi- 
ducts which  would  tend  to  overcome  the  results  of  the  activity 
of  the  ciliated  cells  lining  the  oviduct.  It  is  known  that  the 
male  cell  can  survive  in  the  female  organs  of  generation  for 
several  days,  a  fact  not  difficult  to  understand,  from  the  method 
of  nutrition  of  the  female  cell  (ovum)  ;  for  we  may  suppose  that 
both  elements  are  not  a  little  alike,  as  they  are  both  slightly 
modified  amoeboid  organisms. 

Nervous  Mechanism  — Incidental  reference  has  been  made  to 
the  directing  influence  of  the  nervous  system  over  the  events 
of  reproduction ;  especially  their  subordination  one  to  another 
to  bring  about  the  general  result.  These  may  now  be  consid- 
ered in  greater  detail. 

Most  of  the  processes  in  which  the  nervous  system  takes 
part  are  of  the  nature  of  reflexes,  or  the  result  of  the  automa- 
ticity  (independent  action)  of  the  nerve-centers,  increased  by 
some  afferent  (ingoing)  impressions  along  a  nerve-path.  It  is 
not  always  possible  to  estimate  the  exact  share  each  factor 
takes,  which  must  be  highly  variable.  Certain  experiments 
have  assisted  in  making  the  matter  clear.     It  has  been  found 


134  COMPARATIVE   PHYSIOLOGY. 

that  if,  in  a  female  dog,  the  spinal  cord  he  divided  when  the 
animal  is  still  a  puppy,  menstruation  and  impregnation  may 
occur.  If  the  same  experiment  he  performed  on  a  male  dog, 
erection  of  the  penis  and  ejaculation  of  semen  may  he  caused 
by  stimulation  of  the  penis.  As  the  section  of  the  cord  has  left 
the  hinder  part  of  the  animal's  hody  severed  from  the  brain, 
the  creature  is,  of  course,  unconscious  of  anything  happening 
in  all  the  parts  below  the  section,  of  whatever  nature.  If  the 
nervi  erigentes  (from  the  lower  part  of  the  spinal  cord)  be 
stimulated,  the  penis  is  erected ;  and  if  they  be  cut  this  act  be- 
comes impossible,  either  reflexly  by  experiment  or  otherwise. 
Seminal  emissions,  it  is  well  known,  may  occur  during  sleep, 
and  may  be  associated,  either  as  result  or  cause,  with  voluptu- 
ous dreams.  Putting  all  these  facts  together,  it  seems  reason- 
able to  conclude  that  the  lower  part  of  the  spinal  cord  contains 
the  nervous  machinery  requisite  to  initiate  those  influences 
("impulses)  which,  passing  along  the  nerves  to  the  generative 
organs,  excite  and  regulate  the  processes  which  take  place  in 
them.  In  these,  vascular  changes,  as  we  have  seen,  always 
play  a  prominent  part. 

Usually  we  can  recognize  some  afferent  influence,  either 
from  the  brain  (psychical),  from  the  surface — at  all  events 
from  without  that  part  of  the  nervous  system  (center)  which 
functions  directly  in  the  various  sexual  processes.  It  is  com- 
mon to  speak  of  a  number  of  sexual  centers — as  the  erection 
center,  the  ejaculatory  center,  etc.— but  we  much  doubt  whether 
there  is  such  sharp  division  of  physiological  labor  as  these 
terms  imply,  and  they  are  liable  to  lead  to  misconception ;  ac- 
cordingly, in  the  present  state  of  our  knowledge,  we  prefer  to 
speak  of  the  sexual  center,  using  even  that  term  in  a  somewhat 
broad  sense. 

The  effects  of  stimulation  of  the  sexual  organs  are  not  con- 
fined to  the  parts  themselves,  but  the  ingoing  impulses  set  up 
radiating  outgoing  ones,  which  affect  widely  remote  areas  of 
the  body,  as  is  evident,  especially  in  the  vascular  changes:,  the 
central  current  of  nerve  influence  breaks  up  into  many  streams 
as  a  result  of  the  rapid  and  extensive  rise  of  the  outflowing 
current,  which  breaks  over  ordinary  barriers,  and  takes  paths 
which  are  not  properly  its  own.  Bearing  this  fact  in  mind, 
the  chemical  composition  of  semen,  so  rich  in  proteid  and  other 
material  valuable  from  a  nutritive  point  of  view,  and  consid- 
ering how  the  sexual  appetites  may  engross  the  mind,  it  is  not 


THE   DEVELOPMENT   OF  THE  EMBRYO   ITSELF.    135 

difficult  to  understand  that  nothing  so  quickly  disorganizes  the 
whole  man,  physical,  mental,  and  moral,  as  sexual  excesses, 
whether  by  the  use  of  the  organs  in  a  natural  way,  or  from 
masturbation. 

Nature  has  protected  the  lower  animals  by  the  strong  bar- 
rier of  instinct,  so  that  habitual  sexual  excess  is  with  them  an 
impossibility,  since  the  females  do  not  permit  of  the  approaches 
of  the  male  except  during  the  rutting  period,  which  occurs 
only  at  stated,  comparatively  distant  periods  in  most  of  the 
higher  mammals.  When  man  keeps  his  sexual  functions  in 
subjection  to  his  higher  nature,  they  likewise  tend  to  advance 
his  whole  development. 

Summary. — Certain  changes,  commencing  with  the  ripening 
of  ova,  followed  by  their  discharge  and  conveyance  into  the 
uterus,  accompanied  by  simultaneous  and  subsequent  modifica- 
tions of  the  uterine  mucous  membrane,  constitute,  when  preg- 
nancy occurs,  an  unbroken  chain  of  biological  events,  though 
usually  described  separately  for  the  sake  of  convenience. 
When  impregnation  does  not  result,  there  is  a  retrogression  in 
the  uterus  (menstruation)  and  a  return  to  general  quiescence 
in  all  the  reproductive  organs. 

Parturition  is  to  be  regarded  as  the  climax  of  a  variety  of 
rhythmic  occurrences  which  have  been  gradually  gathering 
head  for  a  long  period.  The  changes  which  take  place  in  the 
placenta  of  a  degenerative  character  fit  it  for  being  cast  off,  and 
may  render  this  structure  to  some  extent  a  foreign  body  before 
it  and  the  foetus  are  finally  expelled,  so  that  these  changes  may 
constitute  one  of  a  number  of  exciting  causes  of  the  increased 
uterine  action  of  parturition.  But  it  is  important  to  regard  the 
whole  of  the  occurrences  of  pregnancy  as  a  connected  series  of 
processes  co-ordinated  by  the  central  nervous  system  so  as  to 
accomplish  one  great  end.  the  development  of  a  new  individual. 

The  nutrition  of  the  ovum  in  its  earliest  stages  is  effected  by 
means  in  harmony  with  its  nature  as  an  amoeboid  organism ; 
nutrition  by  the  cells  of  blood-vessels  is  similar,  while  that  by 
villi  may  be  compared  to  what  takes  place  through  the  agency  of 
similar  structures  in  the  alimentary  canal  of  the  adult  mammal. 

The  circulation  of  the  foetus  puts  it  on  a  par  physiologically 
with  the  lower  vertebrates.  Before  birth  there  is  a  gradual 
though  somewhat  rapid  preparation,  resulting  in  changes  which 
speedily  culminate  after  birth  on  the  establishment  of  the  per- 
manent condition  of  the  circulation  of  extra-uterine  life. 


136  COMPARATIVE   PHYSIOLOGY. 

The  blood  of  the  foetus  (as  in  the  adult)  is  the  great  store- 
house of  nutriment  and  the  common  receptacle  of  all  waste 
products  ;  these  latter  are  in  the  main  transferred  to  the  moth- 
er's blood  indirectly  in  the  placenta;  in  a  similar  way  nutri- 
ment is  imported  from  the  mother's  blood  to  that  of  the  foetus. 
The  placenta  takes  the  place  of  digestive,  respiratory,  and  excre- 
tory organs. 

Coitus  is  essential  to  bring  the  male  and  female  elements 
together  in  the  higher  vertebrates.  The  erection  of  the  penis  is 
owing  to  vascular  changes  taking  place  in  an  organ  composed 
of  erectile  tissue  ;  ejaculation  of  semen  is  the  result  of  the 
peristaltic  action  of  the  various  parts  of  the  sexual  tract,  aided 
by  rhythmical  action  of  certain  striped  muscles.  The  sperma- 
tozoa, which  are  unicellular,  flagellated  (ciliated)  cells,  make  up 
the  essential  part  of  semen  ;  though  the  latter  is  complicated 
by  the  addition  of  the  secretions  of  several  glands  in  connection 
with  the  seminal  tract.  Though  competent  by  their  own  move- 
ments of  reaching  the  ovum  in  the  oviduct,  it  is  probable  that 
the  uterus  and  oviduct  experience  peristaltic  actions  in  a  direc- 
tion toward  the  ovary,  at  least  in  a  number  of  mammals. 

The  lower  part  of  the  spinal  cord  is  the  seat  in  the  higher 
mammals  of  a  sexual  center  or  collection  of  cells  that  l^eceives 
afferent  impulses  and  sends  out  efferent  impulses  to  the  sexual 
organs.  This,  like  all  the  lower  centers,  is  under  the  control  of 
the  higher  centers  in  the  brain,  so  that  its  action  may  be  either 
initiated  or  inhibited  by  the  cerebrum. 


ORGANIC   EVOLUTION   RECONSIDERED. 


Admitting  that  the  theories  of  the  leading  writers  on  the 
subject  have  advanced  us  on  the  way  to  more  complete  views  of 
the  mode  of  origin  of  the  forms  of  the  organic  world,  it  must 
still  be  felt  that  all  theories  yet  propounded  fall  short  of  being 
entirely  satisfactory.  It  seems  to  us  unfortunate  that  the  sub- 
ject has  not  received  more  attention  from  physiologists,  as 
without  doubt,  the  final  solution  must  come  through  that  sci- 
ence which  deals  with  the  properties  rather  than  the  forms  of 
protoplasm  ;  or,  in  other  words,  the  fundamental  principles 
underlying  organic  evolution  are  physiological.  But,  in  the 
unraveling  of  a  subject  of  such  extreme  complexity,  all  sciences 
must  probably  contribute  their  quota  to  make  up  the  truth,  as 
many  rays  of  different  colors  compounded  form  white  light. 
As  with  other  theories  of  the  inductive  sciences,  none  can  be 
more  than  temporary ;  there  must  be  constant  modification  to 
meet  increasing  knowledge.  Conscious  that  any  views  we  our- 
selves advance  must  sooner  or  later  be  modified  as  all  others, 
even  if  acceptable  now,  we  venture  to  lay  before  the  reader  the 
opinions  we  have  formed  upon  this  subject  as  the  result  of  con- 
siderable thought. 

All  vital  phenomena  may  be  regarded  as  the  resultant  of 
the  action  of  external  conditions  and  internal  tendencies.  Amid 
the  constant  change  which  like  involves  we  recognize  two 
things  :  the  tendency  to  retain  old  modes  of  behavior,  and  the 
tendency  to  modification  or  variatiou.  Since  those  impulses 
originally  bestowed  on  matter  when  it  became  living,  must,  in 
order  to  prevail  against  the  forces  from  without,  which  tend  to 
destroy  it,  have  considerable  potency,  the  tendency  to  modifica- 
tion is  naturally  and  necessarily  less  than  to  permanence  of 
form  and  function. 

From  these  principles  it  follows  that  when  an  Amoeba  or 
kindred  organism  divides  after  a  longer  or  shorter  period,  it  is 


138  COMPARATIVE   PHYSIOLOGY. 

not  in  reality  the  same  in  all  respects  as  when  its  existence  be- 
gan, though  we  ruay  be  quite  unable  to  detect  the  changes ;  and 
when  two  infusorians  conjugate,  the  one  brings  to  the  other 
protoplasm  different  in  molecular  behavior,  of  necessity,  from 
having  had  different  experiences.  We  attach  great  importance 
to  these  principles,  as  they  seem  to  us  to  lie  at  the  root  of  the 
whole  matter.  What  has  been  said  of  these  lower  but  inde- 
pendent forms  of  life  applies  to  the  higher.  All  organisms  are 
made  up  of  cells  or  aggregations  of  cells  and  their  products. 
For  the  present  we  may  disregard  the  latter.  When  a  muscle- 
cell  by  division  gives  rise  to  a  new  cell,  the  latter  is  not  identi- 
cally the  same  in  every  particular  as  the  .parent  cell  was  origi- 
nally. It  is  what  its  parent  has  become  by  virtue  of  those  ex- 
periences it  has  had  as  a  muscle-cell  per  se,  and  as  a  member  of 
a  populous  biological  community,  of  the  complexities  of  which 
we  can  scarcely  conceive. 

Now,  as  a  body  at  rest  may  remain  so,  or  may  move  in  a 
certain  direction  according  to  the  forces  acting  upon  it  exactly 
counterbalance  one  another,  or  produce  a  resultant  effect  in  the 
direction  in  which  the  body  moves,  so  in  the  case  of  heredity, 
whether  a  certain  quality  in  the  parent  appears  in  the  offspring, 
depends  on  whether  this  quality  is  neutralized,  augmented,  or 
otherwise  modified  by  any  corresponding  quality  in  the  other 
parent,  or  by  some  opposite  quality,  taken  in  connection  with 
the  direct  influence  of  the  environment  during  development. 

This  assumption  explains  among  other  things  why  acquired 
peculiarities  (the  results  of  accident,  habit,  etc.)  may  or  may 
not  be  inherited. 

These  are  not  usually  inherited  because,  as  is  to  be  expected, 
those  forces  of  the  organism  which  have  been  gathering  head 
for  ages  are  naturally  not  easily  turned  aside.  Again,  we  urge, 
heredity  must  be  more  pronounced  than  variation. 

The  ovum  and  sperm-cell,  like  all  other  cells  of  the  body, 
are  microcosms  representing  the  whole  to  a  certain  extent  in 
themselves — that  is  to  say,  cell  A  is  what  it  is  by  reason  of  what 
all  the  other  millions  of  its  fellows  in  the  biological  republic 
are ;  so  that  it  is  possible  to  understand  why  sexual  cells  repre- 
sent, embody,  and  repeat  the  whole  biological  story,  though  it 
is  not  yet  possible  to  indicate  exactly  how  they  more  than  others 
have  this  power.  This  falls  under  the  laws  of  specialization 
and  the  physiological  division  of  labor  ;  but  along  what  paths 
they  have  reached  this  we  can  not  determine. 


ORGANIC   EVOLUTION   RECONSIDERED.  139 

Strong-  evidence  is  furnished  for  the  above  views  by  the  his- 
tory of  disease.  Scar-tissue,  for  example,  continues  to  repro- 
duce itself  as  such  ;  like  produces  like,  though  in  this  instance 
the  like  is  in  the  first  instance  a  departure  from  the  normal. 
Gout  is  well  known  to  be  a  hereditary  disease  ;  not  only  so,  but 
it  arises  in  the  offspring  at  about  the  same  age  as  in  the  parent, 
which  is  equivalent  to  saying  that  in  the  rhythmical  life  of 
certain  cells  a  period  is  reached  when  they  display  the  behavior, 
physiologically,  of  their  parents.  Yet  gout  is  a  disease  that  can 
be  traced  to  peculiar  habits  of  living  and  may  be  eventually 
escaped  by  radical  changes  in  this  respect — that  is  to  say,  the 
behavior  of  the  cells  leading  to  gout  can  be  induced  and  can  be 
altered  ;  gout  is  hereditary,  yet  eradicable. 

Just  as  gout  may  be  set  up  by  the  formation  of  certain  modes 
of  action  of  the  cells  of  the  body,  so  may  a  mode  of  behavior, 
in  the  nervous  system,  for  example,  become  organized  or  fixed, 
become  a  habit,  and  so  be  transmitted  to  offspring.  It  will  pass 
to  the  descendants  or  not  according  to  the  principles  already 
noticed.  If  so  fixed  in  the  individual  in  which  it  arises  as  to 
predominate  over  more  ancient  methods  of  cell  behavior,  and 
not  neutralized  by  the  strength  of  the  normal  physiological  ac- 
tion of  the  corresponding  parts  in  the  other  parent,  it  will  reap- 
pear. We  can  never  determine  whether  this  is  so  or  not  before- 
hand ;  hence  the  fact  that  it  is  impossible,  especially  in  the  case 
of  man,  whose  vital  processes  are  so  modified  by  his  psychic 
life,  to  predict  whether  acquired  variations  shall  become  heredi- 
tary ;  hence  also  the  irregulai'ity  which  characteiizes  heredity 
in  such  cases  ;  they  may  reappear  in  offspring  or  they  may  not. 
In  viewing  heredity  and  modification  it  is  impossible  to  get  a 
true  insight  into  the  matter  without  taking  into  the  account 
both  the  original  natural  tendencies  of  living  matter  and  the 
influence  of  environment.  We  only  know  of  vital  manifes- 
tations in  some  environment ;  and,  so  far  as  our  experience 
goes,  life  is  impossible  apart  from  the  reaction  of  surround- 
ings. With  these  general  principles  to  guide  us,  we  shall  at- 
tempt a  brief  examination  of  the  leading  theories  of  organic 
evolution. 

First  of  all,  Spencer  seems  to  be  correct  in  regarding  evolu- 
tion as  universal,  and  organic  evolution  but  one  part  of  a 
whole.  No  one  who  looks  at  the  facts  presented  in  every  field 
of  nature  can  doubt  that  struggle  (opposition,  action  and  reac- 
tion) is  universal,  and  that  in  the  organic  world  the  fittest  to  a 


140  COMPARATIVE   PHYSIOLOGY. 

given  environment  survives.  But  Darwin  has  probably  fixed 
his  attention  too  closely  on  this  principle  and  attempted  to  ex- 
plain too  much  by  it,  as  well  as  failed  to  see  that  there  are 
other  deeper  facts  underlying-  it.  Variation,  which  this  author 
scarcely  attempted  to  explain,  seems  to  us  to  be  the  natural  re- 
sult of  the  very  conditions  under  which  living  things  have  an 
existence.  Stable  equilibrium  is  an  idea  incompatible  with  our 
fundamental  conceptions  of  life.  Altered  function  implies  al- 
tered molecular  action,  which  sometimes  leads  to  appreciable 
structural  change.  From  our  conceptions  of  the  nature  of  liv- 
ing matter,  it  naturally  follows  that  variation  should  be  great- 
est, as  has  been  observed,  under  the  greatest  alteration  in  the 
surroundings. 

We  are  but  very  imperfectly  acquainted  as  yet  with  the 
conditions  under  which  life  existed  in  the  earlier  epochs  of  the 
earth's  history.  Of  late,  deep-sea  soundings  and  arctic  explo- 
rations have  brought  surprising  facts  to  light,  showing  that 
living  matter  can  exist  under  a  greater  variety  of  conditions 
than  was  previously  supposed.  Thus  it  turns  out  that  light  is 
not  an  essential  for  life  everywhere.  We  think  these  recent 
revelations  of  unexpected  facts  should  make  us  cautious  in 
assuming  that  life  always  manifested  itself  under  conditions 
closely  similar  to  those  we  know.  Variation  may  at  one  period 
have  been  more  sudden  and  marked  than  Darwin  supposes; 
and  there  does  seem  to  be  rooni  for  such  a  conception  as  the 
"extraordinary  births'"  of  Mivart  implies;  though  we  would 
not  have  it  understood  that  we  think  Darwin's  view  of  slow 
modification  inadequate  to  produce  a  new  species,  we  simply 
venture  to  think  that  he  was  not  justified  in  insisting  so  strongly 
that  this  was  the  only  method  of  Nature;  or,  to  put  it  more 
justly  for  the  great  author  of  the  Origin  of  Species,  with  the 
facts  that  have  accumulated  since  his  time  he  would  scarcely 
be  warranted  in  maintaining  so  rigidly  his  conviction  that 
new  forms  arose  almost  exclusively  by  the  slow  process  he  has 
so  ably  described. 

We  must  allow  a  great  deal  to  use  and  effort,  doubtless,  and 
they  explain  the  origin  of  variations  up  to  a  certain  point,  but 
the  solution  is  only  partial.  Variations  must  arise  as  we  have 
attempted  to  explain,  and  use  and  disuse  are  only  two  of  the 
factors  amid  many.  Correlated  growth,  or  the  changes  in  one 
part  induced  by  changes  in  another,  is  a  principle  which, 
though  recognized  by  Darwin,  Cope,  and  others,  has  not,  we 


ORGANIC   EVOLUTION   RECONSIDERED.  141 

think,  received  the  attention  it  deserves.  To  the  mind  of  the 
physiologist,  all  changes  must  be  correlated  -with  others. 

In  what  sense  has  the  line  that  evolution  has  taken  been 
predetermined  ?  In  the  sense  that  all  things  in  the  universe 
are  unstable,  are  undergoing  change,  leading  to  new  forms  and 
qualities  of  such  a  character  that  they  result  in  a  gradual  prog- 
ress toward  what  our  minds  can  not  but  consider  higher  mani- 
festations of  being. 

The  secondary  methods  according  to  which  this  takes  place 
constitute  the  laws  of  nature,  and  as  wye  learn  from  the  prog- 
ress of  science  are  very  numerous.  The  unity  of  nature  is  a 
reality  toward  which  our  conceptions  are  constantly  leading 
vis.  Evolution  is  a  necessity  of  living  matter  (indeed,  all  matter) 
as  we  view  it. 


THE    CHEMICAL    CONSTITUTION    OF    THE 
ANIMAL  BODY. 


One  visiting  the  ruins  of  a  vast  and  elaborate  building-, 
which  had  been  entirely  pulled  to  pieces,  would  get  an  amount 
of  information  relative  to  the  original  structure  and  uses  of 
the  various  parts  of  the  edifice  largely  in  proportion  to  his  fa- 
miliarity with  architecture  and  the  various  trades  which  make 
that  art  a  practical  success.  The  study  of  the  chemistry  of 
the  animal  body  is  illustrated  by  such  a  case.  Any  attempt 
to  determine  the  exact  chemical  composition  of  living  matter 
must  result  in  its  destruction ;  and  the  amount  of  information 
conveyed  by  the  examination  of  the  chemical  ruins,  so  to  speak, 
will  depend  a  great  deal  on  the  knowledge  already  possessed  of 
chemical  and  vital  processes. 

It  is  in  all  probability  true  that  the  nature  of  any  vital  pro- 
cess is  at  all  events  closely  bound  up  with  the  chemical  changes 
involved  ;  but  we  must  not  go  too  far  in  this  direction.  We  are 
not  yet  prepared  to  say  that  life  is  only  the  manifestation  of 
certain  chemical  and  physical  processes,  meaning  thereby  such 
chemistry  and  physics  as  are  known  to  us  ;  nor  are  we  prepared 
to  go  the  length  of  those  who  regard  life  as  but  the  equivalent 
of  some  other  force  or  forces  ;  as  electricity  may  be  considered 
as  the  transformed  representative  of  so  much  heat  and  vice  versa. 
It  may  be  so,  but  we  do  not  consider  that  this  view  is  warranted 
in  the  present  state  of  our  knowledge. 

On  the  other  hand,  vital  phenomena,  when  our  investiga- 
tions are  pushed  far  enough,  always  seem  to  be  closely  asso- 
ciated with  chemical  action  :  hence  the  importance  to  the  stu- 
dent of  physiology  of  a  sound  knowledge  of  chemical  princi- 
ples. We  think  the  most  satisfactory  method  of  studying  the 
functions  of  an  organ  will  be  found  to  be  that  which  takes  into 
consideration  the  totality  of  the  operations  of  which  it  is  the 
seat,  together  with   its  structure  and  chemical   composition  ; 


CHEMICAL   CONSTITUTION  OP   THE  ANIMAL   BODY.    143 

hence  we  shall  treat  chemical  details  in  the  chapters  devoted  to 
special  physiology,  and  here  give  only  such  an  outline  as  will 
bring  before  the  view  the  chemical  composition  of  the  body  in 
its  main  outlines  ;  and  even  many  of  these  will  gather  a  signifi- 
cance, as  the  study  of  physiology  progresses,  that  they  can  not 
possibly  have  at  the  present. 

Fewer  than  one  third  of  the  chemical  elements  enter  into 
the  composition  of  the  mammalian  body  ;  in  fact,  the  great 
bulk  of  the  organism  is  composed  of  carbon,  hydrogen,  nitro- 
gen, and  oxygen  ;  sodium,  potassium,  magnesium,  calcium, 
sulphur,  phosphorus,  chlorine,  iron,  fluorine,  silicou,  though 
occurring  in  very  small  quantity,  seem  to  be  indispensable  to 
the  living  body  ;  while  certain  others  are  evidently  only  pres- 
ent as  foreign  bodies  or  impurities  to  be  thrown  out  sooner 
or  later.  It  need  scarcely  be  said  that  the  elements  do  not 
occur  as  such  in  the  living  body,  but  in  combination  form- 
ing salts,  which  latter  are  usually  united  with  albuminous 
compounds.  As  previously  mentioned,  the  various  parts  which 
make  up  the  entire  body  of  an  animal  are  composed  of  living 
matter  in  very  different  degrees  ;  hence  we  find  in  such  parts 
as  the  bones  abundance  of  salts,  relative  to  the  proportion  of 
proteid  matter;  a  condition  demanded  by  that  rigidity  without 
which  an  internal  skeleton  would  be  useless,  a  defect  well  illus- 
trated by  that  disease  of  the  bones  known  as  rickets,  in  which 
the  lime-salts  are  insufficient.  It  is  manifest  that  there  may  be 
a  very  great  variety  of  classifications  of  the  compounds  found 
in  the  animal  body  according  as  we  regard  it  from  a  chemical, 
physical,  or  physiological  point  of  view,  or  combine  many 
aspects  in  one  whole.  The  latter  is,  of  course,  the  most  correct 
and  profitable  method,  and  as  such  is  impossible  at  this  stage 
of  the  student's  progress  ;  we  shall  simply  present  him  with  the 
following  outline,  which  will  be  found  both  simple  and  com- 
prehensive. * 

CHEMICAL    CONSTITUTION   OF   THE   BODY. 

Such  food  as  supplies  energy  directly  must  contain  carbon 
compounds. 

Living  matter  or  protoplasm  always  contains  nitrogenous 
carbon  compounds. 


*  Taken  from  the  author's  Outlines  of  Lectures  on  Physiology,  W.  Drys- 
dale  &  Co.,  Montreal. 


144  COMPARATIVE   PHYSIOLOGY. 

In  consequence,  C,  H,  O,  N,  are  the  elements  found  in  great- 
est abundance  in  the  body. 

The  elements  S  and  P  are  associated  with  the  nitrogenous 
carbon  compounds ;  they  also  form  metallic  sulphates  and  phos- 
phates. 

CI  and  F  form  salts  with  the  alkaline  metals  Na,  K,  and  the 
earthy  metals  Ca  and  Mg. 

Fe  is  found  in  hcemoglobin  and  its  derivatives. 

Protoplasm,  when  submitted  to  chemical  examination,  is 
killed.  It  is  then  found  to  consist  of  proteids,  fats,  carbohy- 
drates, salines,  and  extractives. 

It  is  probable  that  when  living  it  has  a  very  complex  mole- 
cule consisting  of  C,  H,  O,  N,  S,  and  P  chiefly. 

Proximate  Principles. 

(a)  Nitrogenous.  -j  Certain  crystalliue  bodies> 

(b)  Non-nitrogenous,  j  ^btfhydrates. 

~    T  (  Mineral  salts. 

2.  Inorganic,  j  Water_ 

Salts. — In  general,  the  salts  of  sodium  are  more  characteris- 
tic of  animal  tissues  and  those  of  potassium  of  vegetable  tissues. 

Na  CI  is  more  abundant  in  the  fluids  of  animals  ;  K  and 
phosphates  more  abundant  in  the  tissues. 
■  Earthy  salts  are  most  abundant  in  the  harder  tissues. 

The  salts  are  probably  not  much,  if  at  all,  changed  in  their 
passage  through  the  body. 

In  some  cases  there  is  a  change  from  acid  to  neutral  or 
alkaline. 

The  salts  are  essential  to  preserve  the  balance  of  the  nutritive 
processes.     Their  absence  leads  to  disease,  e.  g.,  scurvy. 

GENERAL  CHARACTERISTICS  OF  PROTEIDS. 

They  are  the  chief  constituents  of  most  living  tissues,  includ- 
ing blood  and  lymph. 

The  molecule  consists  of  a  great  number  of  atoms  (complex 
constitution),  and  is  formed  of  the  elements  C,  !!,•  N,  0,  S,  and  P. 

All  proteids  are  amorphous. 

All  are  non-diffusible,  the  peptones  excepted. 

They  are  soluble  in  strong  acids  and  alkalies,  with  change  of 
properties  or  constitution. 

In  general,  they  are  coagulated  by  alcohol,  ether,  and  heating. 


CHEMICAL   CONSTITUTION  OF   THE  ANIMAL  BODY.   145 

Coagulated  proteids  are  soluble  only  in  strong  acids  and 
alkalies. 

Classification  and  Distinguishing  Characters  of  Proteids. 

1.  Native  albumins :  Serum  albumin  ;  egg  albumin  ;  solu- 
ble in  water. 

2.  Derived  albumins  (albuminates)  :  Acid  and  alkali  albu- 
min ;  casein  ;  soluble  in  dilute  acids  and  alkalies,  insoluble  in 
water.     Not  precipitated  by  boiling. 

3.  Globulins:  Globulin  (globin)  ;  paraglobulin  ]  myosin; 
fibrinogen.  Soluble  in  dilute  saline  solutions,  and  precipitated 
by  stronger  saline  solutions. 

4.  Peptones :  Soluble  in  water ;  diffusible  through  anima 
membranes;  not  precipitated  by  acids,  alkalies,  or  heat.  De- 
rived from  the  digestion  (peptic,  pancreatic)  of  all  proteids. 

5.  Fibrin  :  Insoluble  in  water  and  dilute  saline  solutions. 
Soluble,  but  not  readily,  in  strong  saline  solutions  and  in  dilute 
acids  and  alkalies. 

CERTAIN  NON-CRYSTALLINE  BODIES. 

The  following  bodies  are  allied  to  proteids,  but  are  not  the 
equivalents  of  the  latter  in  the  food. 

They  are  all  composed  of  C,  H,  N,  O.  Chondrin,  gelatin, 
keratin  have,  in  addition,  S. 

Chondrin  :  The  organic  basis  of  cartilage.  Its  solutions  set 
into  a  firm  jelly  on  cooling. 

Gelatin  :  The  organic  basis  of  bone,  teeth,  tendon,  etc.  Its 
solutions  set  (glue)  on  cooling. 

Elastin :  The  basis  of  elastic  tissue.  Its  solutions  do  not 
set  jelly-like  (gelatinize). 

Mucin  :  From  the  secretion  of  mucous  membranes  ;  precipi- 
tated by  acetic  acid,  and  insoluble  in  excess. 

Keratin  :  Derived  from  hair,  nails,  epidermis,  horn,  feathers. 
Highly  insoluble. 

Nuclein:  Derived  from  the  nuclei  of  cells.  Not  digested 
by  pepsin  ;  contains  P  but  no  S. 

THE   FATS. 

The  fats  are  hydrocarbons  ;  are  less  oxidized  than  the  carbo- 
hydrates ;  are  inflammable  ;  possess  latent  energy  in  a  high 
degree. 

Chemically,  the  neutral  fats  are  glycerides  or  ethers  of  the 

10 


146  COMPARATIVE   PHYSIOLOGY. 

fatty  acids,  i.  e.,  the  acid  radicles  of  the  fatty  acids  of  the  oleic 
and  acetic  series  replace  the  exchangeable  atoms  of  H  in  the 
triatomic  alcohol  glycerine,  e.  g.  : 

Glycerine.  Palmitic  acid.  Glycerine  tripalmitate  or  palmitin. 

i  OH      HO.OC.Ci6H3l  l  O.CO.C16H31 

C3H6  \  OH  +  HO.OC.C15H31  =  C3H5  \  O.CO.C15H31  +  3H20 
( OH      HO.OC.Ci&H31  ( O.CO.Ci5H31 

A  soap  is  formed  by  the  action  of  caustic  alkalies  on  fats,  e.  g. : 

Tripalmitin.  Potassium  palmitate. 

(c,?h%  \  °>  + 3  <K0H>  = 3  j  (g'*Hl,0)o  f  +C,I;  f  o, 

The  soap  may  be  decomposed  by  a  strong  acid  into  a  fatty 
acid  and  a  salt,  e.  g.  : 

ClHslCCK  +  HC1  =  Ci6H,i.COsH  +  KC1. 

Potassium  palmitate.  Palmitic  acid. 

The  fats  are  insoluble  in  water,  but  soluble  in  hot  alcohol, 
ether,  chloroform,  etc. 

The  alkaline  soaps  are  soluble  in  water. 

Most  animal  fats  are  mixtures  of  several  kinds  in  varying 
proportions  ;  hence  the  melting-point  for  the  fat  of  each  species 
of  animal  is  different. 

PECULIAR  FATS. 

Lecithin,  Protagon,  Cerebrin : 

They  consist  of  C,  H,  N,  O,  and  the  first  two  of  P  in  addi- 
tion. 

They  occur  in  the  nervous  tissues. 

CARBOHYDRATES. 

General  formula,  Cm  (H20)„. 

1.  The  Sugars  :  Dextrose,  or  grape-sugar,  C0H12O6  readily 
undergoes  alcoholic  fermentation  ;  less  readily  lactic  fermen- 
tation. 

Lactose,  milk-sugar,  Ci2HMOn  ;  susceptible  of  the  lactic  acid 
fermentation. 

Inosit,  or  muscle-sugar,  C0H,nO0  ;  capable  of  the  lactic  fer- 
mentation. 

Maltose,  C12HM0„,  capable  of  the  alcoholic  fermentation. 
The  chief  sugar  of  the  digestive  process. 

All  the  above  are  much  less  sweet  and  soluble  than  ordinary 
cane-sugar. 


CHEMICAL   CONSTITUTION   OF   THE   ANIMAL   BODY.   147 

2.  The  Starches  :  Glycogen,  CcH10O5,  convertible  into  dex- 
trose. Occurs  abundantly  in  many  foetal  tissues  and  in  the 
liver,  especially  of  the  adult  animal. 

Dextrin,  C6Hjo06,  convertible  into  dextrose.  Soluble  in 
water  ;  intermediate  between  starch  and  dextrose  ;  a  product 
of  digestion. 

Pathological :  Grape-sugar  occurs  in  the  urine  in  diabetes 
mellitus. 

Certain  substances  formed  within  the  body  may  be  regarded 
as  chiefly  waste-products,  the  result  of  metabolism  or  tissue- 
changes. 

They  are  divisible  into  nitrogenous  metabolites  and  non- 
nitrogenous  metabolites. 

Nitrogenous  Metabolites. 

1.  Urea,  uric  acid  and  compounds,  kreatinin,  xanthin,  hypo- 
xanthin  (sarkin),  hippuric  acid,  all  occuring  in  urine. 

2.  Leucin,  tyrosin,  taurocholic,  and  glycocholic  acids,  which 
occur  in  the  digestive  tract. 

3.  Kreatin,  constantly  found  in  muscle,  and  a  few  others  of 
less  constant  occurrence. 

The  above  consists  of  C,  H,  N,  O.  Taurocholic  acid  contains 
also  S. 

The  molecule  in  most  instances  is  complex. 

Non-Nitrogenous  Metabolites. 

These  occur  in  small  quantity,  and  some  of  them  are  secreted 
in  an  altered  form. 

They  included  lactic  and  sarcolactic  acid,  oxalic  acid,  succinic 
acid,  etc. 


PHYSIOLOGICAL   RESEAKCH    AND 
PHYSIOLOGICAL    SEASONING. 


We  propose  in  this  chapter  to  examine  into  the  methods 
employed  in  physiologcial  investigation  and  teaching,  and  the 
character  of  conclusions  arrived  at  hy  physiologists  as  depend- 
ent on  a  certain  method  of  reasoning. 

The  first  step  toward  a  legitimate  conclusion  in  any  one  of 
the  inductive  sciences  to  which  physiology  belongs  is  the  col- 
lection of  facts  which  are  to  constitute  the  foundation  on 
which  the  inference  is  to  be  based.  If  there  be  any  error  in 
these,  a  correct  conclusion  can  not  be  drawn  by  any  reliable 
logical  process.  On  the  other  hand,  facts  may  abound  in  thou- 
sands and  yet  the  correct  conclusion  never  be  reached,  because 
the  method  of  interpretation  is  faulty,  which  is  equivalent  to 
saying  that  the  process  of  inference  is  either  incomplete  or  in- 
correct. The  conclusions  of  the  ancients  in  regard  to  nature 
were  usually  faulty  from  errors  in  both  these  directions  ;  they 
neither  had  the  requisite  facts,  nor  did  they  correctly  interpret 
those  with  which  they  were  conversant. 

Let  us  first  examine  into  the  methods  employed  by  modern 
physiologists,  and  determine  in  how  far  they  are  reliable.  First, 
there  is  the  method  of  direct  observation,  in  which  no  appara- 
tus whatever  or  only  the  simplest  kind  is  employed  ;  thus,  the 
student  may  count  his  own  respirations,  feel  his  own  heart- 
beats, count  his  pulse,  and  do  a  very  great  deal  more  that  will 
be  pointed  out  hereafter;  or  he  may  examine  in  like  manner  an- 
other fellow-being  or  one  of  the  lower  animals.  This  method 
is  simple,  easy  of  application,  and  is  that  usually  employed  by 
the  physician  even  at  the  present  day,  especially  in  private 
practice.  The  value  of  the  results  obviously  depends  on  the 
reliability  of  the  observer  in  two  respects  :  First,  as  to  the  ac- 
curacy, extent,  and  delicacy  of  his  perceptions  ;  and,  secondly, 
on  the  inferences   based  on  these   sense-observations.      Much 


PHYSIOLOGICAL  RESEARCH   AND   REASONING.      149 

must  depend  on  practice —  that  is  to  say,  the  education  of  the 
senses.  The  hand  may  become  a  most  delicate  instrument  of 
observation  ;  the  eye  may  learn  to  see  what  it  once  could  not  ; 
the  ear  to  detect  and  discriminate  what  is  quite  beyond  the 
uncultured  hearing  of  the  many.  But  it  is  one  of  the  most 
convincing  evidences  of  man's  superiority  that  in  every  field  of 
observation  he  has  risen  above  the  lower  animals,  some  of  which 
by  their  unaided  senses  naturally  excel  him.  So  in  this  science, 
instruments  have  opened  up  mines  of  facts  that  must  have  other- 
wise remained  hidden  ;  they  have,  as  it  were,  provided  man 
with  additional  senses,  so  much  have  the  natural  powers  of 
those  he  already  possessed  been  sharpened. 

But  the  chief  value  of  the  results  reached  by  instruments  * 
consists  in  the  fact  that  the  movements  of  the  living  tissues  can 
be  registered ;  i.  e.,  the  great  characteristic  of  modern  physiol- 
ogy is  the  extensive  employment  of  the  graphic  method,  which 
has  been  most  largely  developed  by  the  distinguished  French 
experimenter  Marey.  Usually  the  movements  of  the  point  of 
lever  are  impressed  on  a  smoked  surface,  either  of  glazed 
paper  or  glass,  and  rendered  permanent  by  a  coating  of  some 
material  applied  in  solution  and  drying  quickly,  as  shellac  in 
alcohol.  The  surface  on  which  the  tracing  is  written  may  be 
stationary,  though  this  is  rarely  the  case,  as  the  object  is  to  get 
a  succession  of  records  for  comparison  ;  hence  the  most  used 
form  of  writing  surface  is  a  cylinder  which  may  be  raised  or 
lowered,  and  which  is  moved  around  regularly  by  some  sort  of 
clock-work.  It  follows  that  the  lever  point,  which  is  moved  by 
the  physiological  effect,  describes  curves  of  varying  complexity. 
That  tracings  of  this  or  any  other  character  should  be  of  any 
value  for  the  purposes  of  physiology,  they  must  be  susceptible 
of  relative  measurement  both  for  time  and  space.  This  can  be 
accomplished  only  when  there  is  a  known  base-line  or  abscissa 
from  which  the  lever  begins  its  rise,  and  a  time  record  which  is 
usually  in  seconds  or  portions  of  a  second.  The  first  is  easily 
obtained  by  simply  allowing  the  lever  to  write  a  straight  line 
before  the  physiological  effect  proper  is  recorded.  Time  inter- 
vals are  usually  indicated  by  the  interruptions  of  an  electric 
current,  or  by  the  vibrations  of  a  tuning-fork,  a  pen  or  writer 
of  some  kind  being  in  each  instance  attached  to  the  apparatus 
so  as  to  record  its  movements. 

*  Illustrated  in  the  sections  on  muscle  physiology  and  others. 


150  COMPARATIVE   PHYSIOLOGY. 

As  levers,  in  proportion  to  their  length,  exaggerate  all  the 
movements  imparted  to  them,  a  constant  process  of  correction 
must  be  carried  on  in  the  mind  in  reading  the  records  of  the 
graphic  method,  as  in  interpreting  the  field  of  view  presented 
by  the  microscope. 

The  student  is  epecially  warned  to  carry  on  this  process, 
otherwise  highly  distorted  views  of  the  reality  will  become 
fixed  in  his  own  mind  ;  and  certainly  a  condition  of  ignorance 
is  to  be  preferred  to  such  false  knowledge  as  this  may  become. 
But  it  is  likewise  apparent  that  movements  that  would  without 
such  mechanism  be  quite  unrecognized  may  be  rendered  visible 
and  utilized  for  inference.  There  is  another  source  of  possible 
misconception  in  the  use  of  the  graphic  method.  The  lever  is 
sometimes  used  to  record  the  movements  of  a  column  of  fluid 
(manometer,  Fig.  197),  as  water  or  mercury,  the  inertia  of  which 
is  considerable,  so  that  the  record  is  not  that  of  the  lever  as 
affected  by  the  physiological  (tissue)  movement,  but  that  move- 
ment conveyed  through  a  fluid  of  the  kind  indicated.  Again, 
all  points,  however  delicate,  write  with  some  friction,  and  the 
question  always  arises,  In  how  far  is  that  friction  sufficient  to 
be  a  source  of  inaccuracy  in  the  record  ?  When  organs  are  di- 
rectly connected  with  levers  or  apparatus  in  mechanical  rela- 
tion with  them,  one  must  be  sure  that  the  natural  action  of  the 
organ  under  investigation  is  in  no  way  modified  by  this  con- 
nection. 

From  these  remarks  it  will  be  obvious  that  in  the  graphic 
method  physiologists  possess  a  means  of  investigation  at  once 
valuable  and  liable  to  mislead.  Already  electricity  has  been 
extensively  used  in  the  researches  of  physiologists,  and  it  is  to 
this  and  the  employment  of  photography  that  we  look  in  the 
near  future  for  methods  that  are  less  open  to  the  objections  we 
have  noticed. 

However  important  the  methods  of  physiology,  the  results 
are  vastly  more  so.  We  next  notice,  then,  the  progress  from 
methods  and  observations  to  inferences,  which  we  shall  en- 
deavor to  make  clear  by  certain  cases  of  a  hypothetical  charac- 
ter. Proceeding  from  the  brain  and  entering  the  substance  of 
the  heart,  there  is  in  vertebrates  a  nerve  known  as  the  vagus. 
Suppose  that,  on  stimulating  this  nerve  by  electricity  in  a  rab- 
bit, the  heart  ceases  to  beat,  what  is  the  legitimate  inference  ? 
Apparently  that  the  effect  has  been  due  to  the  action  of  the 
nerve  on  the  heart,  an  action  excited  by  the  use  of  electricity. 


PHYSIOLOGICAL   RESEARCH  AND  REASONING.      151 

This  does  not,  however,  according  to  the  principles  of  a  rigid 
logic,  follow.  The  heart  may  have  ceased  beating  from  some 
cause  wholly  unconnected  with  this  experiment,  or  from  the 
electric  current  escaping  along  the  nerve  and  affecting  some 
nervous  mechanism  within  the  heart,  which  is  not  a  part  of  the 
vagus  nerve  ;  or  it  may  have  been  due  to  the  action  of  the  cur- 
rent on  the  muscular  tissue  of  the  heart  directly,  or  in  some  other 
way.  But  suppose  that  invariably,  whenever  this  experiment 
is  repeated,  the  one  result  (arrest  of  the  beat)  follows,  then  it  is 
clear  that  the  vagus  nerve  is  in  some  way  a  factor  in  the  causa- 
tion. Now,  if  it  could  be  ascertained  that  certain  branches  of 
the  nerve  were  distributed  to  the  heart-muscle  directly,  and  that 
stimulation  of  these  gave  rise  to  arrest  of  the  cardiac  pulsation, 
then  would  it  be  highly  probable,  though  not  certain,  that  there 
was  in  the  first  instance  no  intermediate  mechanism  ;  while 
this  inference  would  become  still  more  probable  if  in  hearts 
totally  without  any  such  nervous  apparatus  whatever,  such  a 
result  followed  on  stimulation  of  the  vagus.  Suppose,  further, 
that  the  application  of  some  drug  or  poison  to  the  heart  pro- 
vided with  special  nervous  elements  besides  the  vagus  termi- 
nals prevented  the  effect  before  noticed  on  stimulating  the 
vagus,  while  a  like  result  followed  under  similar  circumstances 
in  those  forms  of  heart  unprovided  with  such  nervous  struct- 
ures, there  would  be  additional  evidence  in  favor  of  the  view 
that  the  result  we  are  considering  was  due  solely  to  some  action 
of  the  vagus  nerve ;  while,  if  arrest  of  the  heart  followed  in  the 
first  case  but  not  in  the  second,  and  this  result  were  invariable, 
there  would  be  roused  the  suspicion  that  the  action  of  the 
vagus  was  not  direct,  but  through  the  nervous  structures  with- 
in the  heart  other  than  vagus  endings.  And  if,  again,  there  were 
a  portion  of  the  rabbit's  heart  to  which  there  were  distributed 
this  intrinsic  nervous  supply,  which  on  stimulation  directly 
was  arrested  in  its  pulsation,  it  would  be  still  more  probable 
that  the  effect  in  the  first  instance  we  have  considered  was  clue 
to  these  structures,  and  only  indirectly  to  the  vagus.  But  be  it 
observed,  in  all  these  cases  there  is  only  probability.  The  con- 
clusions of  physiology  never  rise  above  probability,  though  this 
may  be  so  strong  as  to  be  practically  equal  in  value  to  absolute 
certainty.  Would  it  be  correct,  from  any  or  all  the  experi- 
ments we  have  supposed  to  have  been  made,  to  assert  that  the 
vagus  was  the  arresting  (inhibitory)  nerve  of  the  heart  ?  All 
hearts  thus  far  examined  have  much  in  common  in  structure 


152  COMPARATIVE   PHYSIOLOGY. 

and  function,  and  in  so  far  is  the  above  generalization  probable. 
Such  a  statement  would,  however,  be  far  from  that  degree  of 
probability  which  is  possible,  and  should  therefore  not  be  ac- 
cepted till  more  evidence  has  been  gathered.  The  mere  resem- 
blance in  form  and  general  function  does  not  suffice  to  meet  the 
demands  of  a  critical  logic.  Such  a  statement  as  the  above  would 
not  necessarily  apply  to  the  hearts  of  all  vertebrates  or  even  all 
rabbits,  if  the  experiments  had  been  conducted  on  one  animal 
alone,  for  the  result  might  be  owing  to  a  mere  idiosyncrasy  of 
the  rabbit  under  observation.  The  further  we  depart  from  the 
group  of  animals  to  which  the  creature  under  experiment  be- 
longs, the  less  is  the  probability  that  our  generalizations  for 
the  one  class  will  apply  to  another.  It  will,  therefore,  be  seen 
that  wide  generalizations  can  not  be  made  with  that  amount  of 
certainty  which  is  attainable  until  experiments  shall  have  be- 
come very  numerous  and  widely  extended.  A  really  broad  and 
sound  j)hysiology  can  only  be  constructed  when  this  science 
has  become  much  more  comparative — that  is,  extended  to  many 
more  groups  and  sub-groups  of  animals  than  at  present. 

We  have  incidently  alluded  throughout  the  work  to  the 
teaching  of  disease.  "  Disease  "  is  but  a  name  for  disordered 
function.  One  viewing  a  piece  of  machinery  for  the  first  time 
in  improper  action  might  draw  conclusions  with  comparative 
safety,  provided  he  had  a  knowledge  of  the  correct  action  of 
similar  machines.  Our  experience  gives  us  a  certain  knowl- 
edge of  the  functions  of  our  own  bodies.  By  ordinary  observa- 
tion and  by  experiment  on  other  animals  we  get  additional 
data,  which,  taken  with  the  disordered  action  resulting  from 
gross  or  molecular  injury  (disease),  gives  a  basis  for  certain 
conclusions  as  to  the  normal  functions  of  the  human  body  or 
those  of  lower  animals.  This  information  is  especially  valu- 
able in  the  case  of  man,  since  he  can  report  with  a  fair  de- 
gree of  reliability,  in  most  diseased  conditions,  his  own  sensa- 
tions. 

It  is  hoped  that  this  brief  treatment  of  the  methods  and 
logic  of  physiology  will  suffice  for  the  present.  Throughout 
the  work  they  will  be  illustrated  in  every  chapter,  though  not 
always  with  distinct  references  to  the  nature  of  the  intellectual 
process  followed. 

Summary. — There  are  two  methods  of  physiological  observa- 
tion, the  direct  and  the  indirect.  The  first  is  the  simplest,  and 
is  valuable  in  proportion  to   the   accuracy  and   delicacy  and 


PHYSIOLOGICAL   RESEARCH  AND   REASONING.      153 

range  of  the  observer  ;  the  latter  implies  the  use  of  apparatus, 
and  is  more  complex,  more  extended,  more  delicate,  and  precise. 
It  is  usually  employed  with  the  graphic  method,  which  has  the 
advantage  of  recording  and  thus  preserving  movements  which 
correspond  with  more  or  less  exactness  to  the  movements  of 
tissues  or  organs.  It  is  valuable,  but  liable  to  errors  in  record- 
ing and  in  interpretation. 

The  logic  of  physiology  is  that  of  the  inductive  sciences.  It 
proceeds  from  the  special  to  the  general.  The  conclusions  of 
physiology  never  pass  beyond  extreme  probability,  which,  in 
some  cases,  is  practically  equal  to  certainty.  It  is  especially 
important  not  to  make  generalizations  that  are  too  wide. 


THE   BLOOD. 


It  is  a  matter  of  common  observation  that  the  loss  of  the 
whole,  or  a  very  large  part,  of  the  blood  of  the  body  entails 
death ;  while  an  abundant  haemorrhage,  or  blood-disease  in  any 
of  its  forms,  causes  great  general  weakness. 

The  student  of  embryology  is  led  to  inquire  as  to  the  neces- 
sity for  the  very  early  appearance  and  the  rapid  development 
of  the  blood- vascular  system  so  prominent  in  all  vertebrates. 

An  examination  of  the  means  of  transit  of  the  blood,  as 
already  intimated,  reveals  a  complicated  system  of  tubes  dis- 
tributed to  every  organ  and  tissue  of  the  body.  These  facts 
would  lead  one  to  suppose  that  the  blood  must  have  a  tran- 
scendent importance  in  the  economy,  and  such,  upon  the  most 
minute  investigation,  proves  to  be  the  case.  The  blood  has 
been  aptly  compared  to  an  internal  world  for  the  tissues,  an- 
swering to  the  external  world  for  the  organism  as  a  whole. 
This  fluid  is  the  great  storehouse  containing  all  that  the  most 
exacting  cell  can  demand  ;  and,  further,  is  the  temporary  re- 
ceptacle of  all  the  waste  that  the  most  busy  cell  requires  to  dis- 
charge. Should  such  a  life-stream  cease  to  flow,  the  whole  vital 
machinery  must  stop — death  must  ensue. 

Comparative. — It  will  prove  more  scientific  and  generally 
satisfactory  to  regard  the  blood  as  a  tissue  having  a  fluid  and 
flowing  matrix,  in  which  flow  cellular  elements  or  corpuscles — 
a  view  of  the  subject  that  is  less  startling  when  it  is  remem- 
bered that  the  greater  part  of  the  protoplasm  which  makes  up 
the  other  tissues  of  the  body  is  of  a  semifluid  consistence.  In 
all  animals  possessing  blood,  the  matrix  is  a  clear,  usually  more 
or  less  colored  fluid.  Among  invertebrates  the  color  may  be 
pronounced  :  thus,  in  cephalopods  and  some  crustaceans  it  is 
blue,  but  in  most  groups  of  animals  and  all  vertebrates  the 
matrix  is  either  colorless  or  more  commonly  of  some  slight 
tinge  of  yellow.      Invertebrates   with   few  exceptions  possess 


THE   BLOOD. 


155 


only  colorless  corpuscles,  but  all  vertebrates  have  colored  cells 
which  invariably  outnumber  the  other  variety,  and  display 
forms  and  sizes 
which  are  sufficient- 
ly constant  to  be 
characteristic.  In  all 
groups  below  mam- 
mals the  colored  cor- 
puscles are  oval, 
mostly  biconvex, 
and  nucleated  dur- 
ing all  periods  of  the 
animal's  existence  ; 
in  mammals  they  are 
circular  biconcave 
disks  (except  in  the 
camel  tribe,  the  cor- 
puscles of  which  are 
oval),  and  in  post- 
embryonic  life  with- 
out a  nucleus  ;  nor 
do  they  possess  a 
cell-wall.      The   red 

cells  vary  in  size  in  different  groups  and  sub-groups  of  animals, 
being  smaller  the  higher  the  place  the  animal  occupies,  as  a 

general  rule ;  thus,  they 


are  very  large  in  verte- 
brates below  mammals, 
in  some  cases  being  al- 
most visible  to  the  un- 
aided eye,  while  in  the 
whole  class  of  mam- 
mals they  are  very  mi- 
nute ;  their  numbers 
also  in  this  group  are 
vastly  greater  than  in 
others  lower  in  the 
scale. 

The  average  size  in 
man  is  ^5  inch  ('0077 
mm.)  and  the  nnmber 
in   a   cubic   millimetre 


Fig.  139. — Leucocytes  of  human  blood,  showing  amoe- 
boid movements  (Landois).  These  movements  are 
not  normally  in  the  blood-vessels  so  marked  as  pic- 
tured here,  so  that  the  figure  represents  an  extreme 
case. 


Fia.  140.- 


-Photograph  of  colored  corpuscles  of  frog. 
1  x"370.     (After  Flint.). 


156 


COMPARATIVE   PHYSIOLOGY. 


of  the  blood  about  5,000,000  for  the  male  and  500,000  less  for 
the  female,  which  would  furnish  about  250,000,000,000  in  a 
pound  of  blood.  It  will  be  understood  that  averages  only  are 
spoken  of,  as  all  kinds  of  variations  occur,  some  of  which  will 
be  referred  to  later,  and  their  significance  explained.  The  size 
of  the  corpuscles  in  the  domestic  animals  is  variable — a  matter 
of  importance  when  transfusion  of  blood  is  under  consideration. 
Under  the  microscope  the  blood  of  vertebrates  is  seen  to  owe 
its  color  to  the  cells  chiefly,  and,  so  far  as  the  red  goes,  almost 

wholly.  Corpuscles 
when  seen  singly  are 
never  of  the  deep  red, 
however,  of  the  blood 
as  a  whole,  but  rather 
a  yellowish  red,  the 
tinge  varying  some- 
what with  the  class  of 
animals  from  which 
the  specimen  has  been 
taken. 

Certain  other  mor- 
phological elements 
found  in  mammalian 
blood  deserve  brief 
mention,  though  their 
significance  is  as  yet 
a  matter  of  much  dis- 
pute. 

1.  The  blood-plates 
(plaques,  hcematoblasts,  third  element),  very  small,  colorless, 
biconcave  disks,  which  are  deposited  in  great  numbers  on  any 
thread  or  similar  foreign  body  introduced  into  the  circulation, 
and  rapidly  break  up  when  blood  is  shed. 

2.  On  a  slide  of  blood  that  has  been  prepared  for  some  little 
time,  aggregations  of  very  minute  granules  (elementary  gran- 
ules) may  be  seen.  These  are  supposed  to  represent  the  disin- 
tegrating protoplasm  of  the  corpuscles. 

The  pale  or  colorless  corpuscles  are  very  few  in  number  in 
mammals  compared  with  the  red,  there  being  on  the  average 
only  about  1  in  400  to  GOO,  though  they  become  much  more 
numerous  after  a  meal.  They  are  granular  in  appearance,  and 
possess  one  or  more  nuclei,  which  are  not,  however,  readily 


Fig.  141.— Corpuscles  from  human  subject  (Pnnke). 
A  few  colorless  corpuscles  are  seen  among  the  col- 
ored disks,  which  are  many  of  them  arranged  in 
rouleaux. 


THE  BLOOD. 


157 


seen  in  all  cases  without  the  use  of  reagents.     They  are  charac- 
terized by  greater  size,  a  globular  form,  the  lack  of  pigment, 


© 

s 


Fia.  142.— Blood-plaques  and  their  derivatives  (Landois,  after  Bizzozero  and  Laker). 
1,  red  blood-corpuscles  on  the  flat;  2,  from  the  side;  3,  unchanged  blood-plaques; 
4,  lymph-corpuscle  surrounded  with  blood-plaques;  5,  blood-plaques  variously 
altered;  6,  lymph-corpuscle  with  two  masses  of  fused  blood-plaques  and  threads 
of  fibrin;  7,  group  of  blood-plaques  fused  or  run  together;  8,  similar  small  mass 
of  partially  dissolved  blood-plaques  with  fibrils  of  fibrin. 

and  the  tendency  to  amoeboid  movements,  which  latter  may  be 
exaggerated  in  disordered  conditions  of  the  blood,  or  when  the 
blood  is  withdrawn  and  observed  under  artificial  conditions. 
It  will  be  understood  that  these  cells  (leucocytes)  are  not  con- 
fined to  the  blood,  but  abound  in  lymph  and  other  fluids. 
They  are  the  representatives  of  the  primitive  cells  of  the  em- 
bryo, as  is  shown  by  their  tendency  (like  ova)  to  throw  out 
processes,  develop  into  higher  forms,  etc.  In  behavior  they 
strongly  suggest  Amoeba  and  kindred  forms. 

We  may,  then,  say  that  in  all  invertebrates  the  blood,  when 
it  exists,  consists  of  a  plasma  (liquor  sanguinis),  in  which  float 
the  cellular  elements  which  are  colorless  ;  and  that  in  verte- 
brates in  addition  there  are  colored  cells  which  are  always  nu- 
cleated at  some  period  of  their  existence.  The  colorless  cells 
are  globular  masses  of  protoplasm,  containing  one  or  more 
nuclei,  and  with  the  general  character  of  amoeboid  organisms. 


158 


COMPARATIVE  PHYSIOLOGY. 


bx: 


The  History  of  the  Blood-Cells. 

We  have  already  seen  that  the  blood  and  the  vessels  in 
which  it  flows  have  a  common  origin  in  the  mesoblastic  cells  of 
the  embryo  chick  ;  the  same  applies  to  mammals  and  lower 
groups.  The  main  facts  may  be  grouped  under  two  head- 
ings :  1.  Development  of  tbe  blood-corpuscles  during  embry- 
onic life.  2.  Development  of  the  corpuscles  in  post-embryonic 
life.  The  origin  and  fate  of  the  corpuscles,  especially  of  the 
colored  variety,  have  been  the  subject  of  much  discussion. 

The  best  established 
facts  are  stated  in  the 
summary  below,  while 
they  are  illustrated  by 
the  accompanying  fig- 
ures. 

The  colorless  cells 
of  the  blood  first  arise 
as  migrated  uu differen- 
tiated remnants  of  the 
eai'ly  embryonic  cell 
colonies.  That  they  re- 
main such  is  seen  by 
their  physiological  be- 
havior, to  be  considered 
a  little  later.  Afterward 
they  are  chiefly  pro- 
duced from  a  peculiar 
form  of  connective  tis- 
sue known  as  leucocy- 
tenic,  and  which  is 
gathered  into  organs 
(lymphatic  glands),  the 
chief  function  of  which  is  to  produce  these  cells,  though  this 
tissue  is  rather  widely  distributed  in  the  mammalian  body  in 
other  forms  than  these. 

Summary. — The  student  may,  with  considerable  certainty, 
consider  the  colorless  corpuscle  of  the  blood  as  the  most  primi- 
tive; the  red,  derived  either  from  the  white  or  some  form  of 
more  specialized  cell  ;  the  nucleated,  as  the  earlier  and  more 
youthful  form  of  the  colored  corpuscle,  which  may  in  some 
groups  of  vertebrates  be  replaced  by  a  more  specialized  (or  de- 


Fig.  143.— Surface  view  from  below  of  a  small  por- 
tion of  posterior  end  of  pellucid  area  of  a  cliick 
of  thirty-six  hours,  1  x  400  (Foster  and  Balfour). 
b.  c,  blood-corpuscles  ;  a,  nuclei,  which  subse- 
quently become  nuclei  of  cells  forming  walls  of 
blood-vessels ;  p.  pr,  protoplasmic  processes, 
containing  nuclei  with  large  nucleoli,  n. 


THE   BLOOD. 


159 


graded  ?)  non-nucleated  form  mostly  derived  directly  from  the 
former  ;  that  in  the  first  instance  the  blood-vessels  and  blood 


©     0    ©    | 


Fig.  144. 


Fig.  145. 


Fig.  146. 


a 


Fig.  147. 


Fig.  148. 


Fig.  144.— Cell  elements  of  red  marrow,  a,  large  granular  marrow  cells;  b,  smaller, 
more  vesicular  cells;  c.  free  nuclei,  or  small  lymphoid  cells,  some  of  which  may 
be  even  surrounded  with  a  delicate  rim  of  protoplasm;  d,  nucleated  red  corpuscles 
of  the  bone  marrow. 

Fig.  145. — Nucleated  red  cells  of  marrow,  illustrating  mode  of  development  into  the 
ordinary  non-nucleated  red  corpuscles,  a.  common  forms  of  the  colored  nucleated 
cells  of  red  marrow;  b,  1.  2,  3.  gradual  disappearance  of  the  nucleus;  c,  large  non- 
nucleated  red  corpuscle  resembling  2  and  3  of  b  in  all  respects  save  in  the  absence 
of  any  trace  of  nucleus. 

Fig.  146.— Nucleated  red  corpuscles,  illustrating  the  migration  of  the  nucleus  from  the 
cell,  a  process  not  unfrequently  seen  in  the  red  marrow. 

Fig.  147.— Blood  of  human  embryo  of  four  months.  «,  1, 2,  3. 4,  nucleated  red  corpus- 
cles. In  4  the  same  granular  disintegrated  appearance  of  the  nucleus  as  is  noted 
in  marrow  cells,    b.  1.  microcyte;  2,  megalocyte;  3,  ordinary  red  corpuscle. 

Fig.  148.— From  spleen.  1,  blood-plaques,  colorless  and  varying  a  little  in  size;  2,  two 
microcytcs  of  a  deep-red  color;  3.  two  ordinary  red  corpuscles;  4,  a  solid,  translu- 
cent, lymphoid  cell  or  free  nucleus.     (Figs.  144-148  after  Osier.) 

arise  simultaneously  in  the  mesohlastic  embryonic  tissue  ;  that 
such  an  organ  may  exist  after  birth,  either  normally  in  some 
mammals  or  under  unusual  functional  need  ;  that  the  red  mar- 
row is  the  chief  birthplace  of  colored  cells  in  adult  life  ;  that 


160  COMPARATIVE   PHYSIOLOGY. 

the  spleen,  liver,  lymphatic  glands,  and  other  tissues  of  similar 
structure  contribute  in  a  less  degree  to  the  development  of  the 
red  corpuscles ;  and  that  the  last  mentioned  organs  are  the  chief 
producers  of  the  colorless  amoeboid  blood-cells. 

Finally,  it  is  well  to  remember  that  Nature's  resources  in 
this,  as  in  many  other  cases,  are  numerous,  and  that  her  mode 
of  procedure  is  not  invariable ;  and  that,  if  one  road  to  an  end 
is  blocked,  another  is  taken. 

The  Decline  and  Death  of  the  Blood-Cells.— The  blood  cor- 
puscles, like  other  cells,  have  a  limited  duration,  with  the  usual 
chapters  in  a  biological  history  of  rise,  maturity,  and  decay. 
There  is  reason  to  believe  that  the  red  cells  do  not  live  longer 
than  a  few  weeks  at  most.  The  red  cells,  in  various  degrees  of 
disorganization,  have  been  seen  within  the  white  cells  {phagocy- 
tes), and  the  related  cells  of  the  spleen,  liver,  bone-marrow,  etc. 
In  fact,  these  cells,  by  virtue  of  retained  ancestral  {amoeboid) 
qualities,  have  devoured  the  weakened,  dying  red  cells.  It  seems 
to  be  a  case  of  survival  of  the  fittest.  It  is  further  known  that 
abundance  of  pigment  containing  iron  is  found  in  both  spleen 
and  liver  ;  and  there  seems  to  be  no  good  reason  for  doubting 
that  the  various  pigments  of  the  secretions  of  the  body  {urine, 
bile,  etc.)  are  derived  from  the  universal  pigment  of  the  blood. 
These  coloring  matters,  then,  are  to  be  regarded  as  the  excreta 
in  the  first  instance  of  cells  behaving  like  amceboids,  and  later 
as  the  elaborations  of  certain  others  in  the  kidney  and  else- 
where, the  special  function  of  which  is  to  get  rid  of  waste  prod- 
ucts. The  birth-rate  and  the  death-rate  of  the  blood-cells  must 
be  in  close  relation  to  each  other  in  health  ;  and  some  of  the 
gravest  disturbances  arise  from  decided  changes  in  the  normal 
proportions  of  the  cells  {anaimia,  leucocyt hernia). 

Both  the  red  and  white  corpuscles  show,  like  all  other  cells 
of  the  organism,  alterations  corresponding  to  changes  in  the 
surrounding  conditions.  The  blood  may  be  withdrawn  and  its 
cells  more  readily  observed  than  those  of  most  tissues  ;  so  that 
the  study  of  the  influence  of  temperature,  feeding  of  the  leuco- 
cytes, and  the  action  of  reagents  in  both  classes  of  cells  is  both 
of  practical  importance  and  theoretic  interest,  and  will  well  re- 
pay the  student  for  the  outlay  in  time  and  labor,  if  attention  is 
directed  chiefly  to  the  results  and  the  lessons  they  convey,  and 
not,  as  too  commonly  happens,  principally  to  the  methods  of 
manipulation. 

The  Chemical  Composition  of  the  Blood,— Blood  has  a  decided 


THE  BLOOD.  161 

but  faint  alkaline  reaction,  owing1  chiefly  to  the  presence  of 
sodium  salts,  a  saline  taste,  and  a  faint  odor  characteristic  of 
the  animal  group  to  which  it  belongs,  owing  probably  to  volatile 
fatty  acids.  The  specific  gravity  of  human  blood  varies  between 
1045  and  1075,  with  a  mean  of  1055  ;  the  specific  gravity  of  the 
corpuscles  being  about  1105  and  of  the  plasma  1027.  This  dif- 
ference explains  the  sinking  of  the  corpuscles  in  blood  with- 
drawn from  the  vessels  and  kept  quiet.  Much  the  same  diffi- 
culties are  encountered  in  attempts  at  the  exact  determination 
of  the  chemical  composition  of  the  blood,  as  in  the  case  of  other 
living  tissues.  Plasma  alters  its  physical  and  its  chemical  com- 
position, to  what  extent  is  not  exactly  known,  when  removed 
from  the  body. 

Composition  of  Serum.— The  fluid  remaining  after  coagula- 
tion of  the  blood  can,  of  course,  be  examined  chemically  with 
considerable  thoroughness  and  confidence. 

By  far  the  greater  part  of  serum  consists  of  water ;  thus,  it 
has  been  estimated  that  of  100  parts  the  following  statement  will 
represent  fairly  well  the  proportional  composition : 

Water 90  parts ; 

Proteids 8  to  9      " 

Salines,  fats,  and  extractives  (small  in 
quantity  and  not  readily  obtained 
free) 1  to  2  parts. 

The  proteids  are  made  up  of  two  substances  which  can  be 
distinguished  by  solubility,  temperature  at  which  coagulation 
occurs,  etc.,  known  as  paraglobulin  and  serum-albumen,  and 
which  may  exist  in  equal  amount. 

It  is  not  possible,  of  course,  to  say  whether  these  substances 
exist  as  such  in  the  living  blood-plasma  or  not. 

The  fats  are  very  variable  in  quantity  iu  serum,  depend- 
ing on  a  corresponding  variability  in  the  plasma,  in  which 
they  would  be  naturally  found  in  greatest  abundance  after  a 
meal.  They  exist  as  neutral  stearin,  palmitin,  olein,  and  as 
soaps. 

The  principal  extractives  found  are  urea,  creatin  and  allied 
bodies,  sugar,  and  lactic  acid.  Serum  in  most  animals  contains 
more  of  sodium  salts  than  the  corpuscles,  while  the  latter  in 
man  and  some  other  mammals  contain  a  preponderating  quan- 
tity of  potassium  compounds. 

The  principal  salts  of  serum  are  sodium  chloride,  sodium  bi- 
carbonate, sodium  sulphate  and  phosphate ;  in  smaller  quantity, 
11 


162  COMPARATIVE  PHYSIOLOGY. 

also  phosphate  of  calcium  and  magnesium,  with  rather  more 
of  potassium  chloride. 

It  is  highly  prohahle  that  this  proportion  also  represents 
moderately  well  the  composition  of  plasma,  which  is,  of  course, 
from  a  physiological  point  of  view,  the  important  matter. 

The  Composition  of  the  Corpuscles.— Taken  together,  the  dif 
ferent  forms  of  blood-cells  make  up  from  one  third  to  nearly 
one  half  the  weight  of  the  blood,  and  of  this  the  red  corpuscles 
may  be  considered  as  constituting  nearly  the  whole. 

The  colorless  cells  are  known  to  contain  fats  and  glycogen, 
which,  with  salts,  we  may  believe  exist  in  the  living  cells,  and, 
in  addition  to  the  proteids,  into  which  protoplasm  resolves  it- 
self upon  the  disorganization  that  constitutes  its  dying,  lecithin, 
protagon,  and  other  extractives. 

The  prominent  chemical  fact  connected  with  the  red  corpus- 
cles is  their  being  composed  in  great  part  of  a  peculiar  colored 
proteid  compound  containing  iron. 

This  will  be  fully  considered  later:  but,  in  the  mean  time 
we  may  state  that  the  haemoglobin  is  itself  infiltrated  into  the 
meshes  or  framework  (stroma)  of  the  corpuscle,  which  latter 
seems  to  be  composed  of  a  member  of  the  globulin  class,  so  well 
characterized  by  solubility  in  weak  saline  solutions. 

The  following  tabular  statement  represents  the  relative  pro- 
portions in  100  parts  of  the  dried  organic  matter  of  the  red  cor- 
puscles : 

Haemoglobin 90  54 

Proteids 8-67 

Lecithin. 0"54 

Cholesterin 0-25 


100-00 
The  quantity  of  salts  is  very  small,  less  than  one  per  cent 
(inorganic). 

So  much  for  the  results  of  our  analyses ;  but  when  we  con- 
sider the  part  the  blood  plays  in  the  economy  of  the  body,  it 
must  appear  that,  since  the  life-work  of  every  cell  expresses  it- 
self through  this  fluid,  both  as  to  what  it  removes  and  what  it 
adds,  the  blood  can  not  for  any  two  successive  moments  be  of 
precisely  the  same  composition ;  yet  the  departures  from  a  nor- 
mal standard  must  be  kept  within  very  narrow  limits,  other- 
wise derangement  or  possibly  death  results.  We  think  that, 
before  we  have  concluded  the  study  of  the  various  organs  of 


THE  BLOOD.  163 

the  body,  it  will  appear  to  the  student,  as  it  does  to  the  writer, 
that  it  is  highly  probable  that  there  are  great  numbers  of  com- 
pounds hi  the  blood,  either  of  a  character  unknown  as  yet  to 
our  chemistry,  or  in  such  small  quantity  that  they  elude  detec- 
tion by  our  methods ;  and  we  may  add  that  we  believe  the  same 
holds  for  all  the  fluids  of  the  body.  The  complexity  of  vital 
processes  is  great  beyond  our  comprehension. 

It  must  be  especially  borne  in  mind  that  all  the  pabulum 
for  every  cell,  however  varied  its  needs,  can  be  derived  from 
the  blood  alone ;  or,  as  we  shall  show  presently,  strictly  speak- 
ing from  the  lymph,  a  sort  of  middle-man  between  the  blood 
and  the  tissues. 

The  Quantity  and  the  Distribution  of  the  Blood.— The  rela- 
tive quantities  of  blood  in  different  parts  of  the  body  have  been 
estimated  to  be  as  follows: 

Liver one  fourth. 

Skeletal  muscles "        " 

Heart,  lungs,  large  arteries,  and  veins.     "        " 
Other  structures ...     "        " 

The  significance  of  this  distribution  will  appear  later. 

The  Coagulation  of  the  Blood.— When  blood  is  removed 
from  its  accustomed  channels,  it  undergoes  a  marked  chemical 
and  physical  change,  termed  clotting  or  coagulation.  In  the 
case  of  most  vertebrates,  almost  as  soon  as  the  blood  leaves  the 
vessels  it  begins  to  thicken,  and  gradually  acquh'es  a  consistence 
that  may  be  compared  to  that  of  jelly,  so  that  it  can  no  longer 
be  poured  from  the  containing  vessel.  Though  some  have  rec- 
ognized different  stages  as  distinct,  and  named  them,  we  think 
that  an  unprejudiced  observer  might  fail  to  see  that  there  were 
any  well-marked  appearances  occurring  invariably  at  the  same 
moment,  or  with  resting  stages  in  the  process,  as  with  the  devel- 
opment of  ova. 

After  coagulation  has  reduced  the  blood  to  a  condition  in 
which  it  is  no  longer  diffluent,  minute  drops  of  a  thin  fluid 
gradually  show  themselves,  exuding  from  the  main  mass,  faintly 
colored,  but  never  red,  if  the  vessel  in  which  the  clot  has 
formed  has  been  kept  quiet  so  that  the  red  corpuscles  have  not 
been  disturbed ;  and  later  it  may  be  noticed  that  the  main  mass 
is  beginning  to  sink  in  the  center  {cupping) ;  and  in  the  blood 
of  certain  animals,  as  the  horse,  which  clots  slowly,  the  upper 
part  of  the  coagulum  (cr  assume  ntum)  appears  of  a  lighter 
color,  owing,  as  microscopic  examination  shows,  to  the  relative 


164  COMPARATIVE   PHYSIOLOGY. 

fewness  of  red  corpuscles.  This  is  the  huffy -coat,  or,  as  it  oc- 
curs in  inflammatory  conditions  of  the  hlood,  was  termed  by 
older  writers,  the  crusta  plilogistica.  It  is  to  be  distinguished 
from  the  lighter  red  of  certain  parts  of  a  clot,  often  the  result  of 
greater  exposure  to  the  air  and  more  complete  oxidation  in  con- 
sequence. The  white  blood-cells,  being  lighter  than  the  red,  are 
also  more  abundant  in  the  upper  part  of  the  clot  (buffy-coat). 
If  the  coagulation  of  a  drop  of  blood  withdrawn  from  one's 
own  finger  be  watched  under  the  microscope,  the  red  corpuscles 
may  be  seen  to  run  into  heaps,  like  rows  of  coins  lying  against 
each  other  (rouleaux,  Fig.  141),  and  threads  of  the  greatest 
fineness  are  observed  to  radiate  throughout  the  mass,  gradually 
increasing  in  number,  and,  at  last,  including  the  whole  in  a 
meshwork  which  slowly  contracts.  It  is  the  formation  of  this 
fibrin  which  is  the  essential  factor  in  clotting;  the  inclusion  of 
the  blood-cells  and  the  extrusion  of  the  serum  naturally  result- 
ing from  its  formation  and  contraction. 

The  great  mass  of  every  clot  consists,  however,  of  corpuscles ; 
the  quantity  of  fibrin,  though  variable,  not  amounting  to  more 
usually  than  about  '2  per  cent  in  mammals.  Tbe  formation  of 
the  clot  does  not  occupy  more  than  a  few  minutes  (two  to  seven) 
in  most  mammals,  including  man,  but  its  contraction  lasts  a 
very  considerable  time,  so  that  serum  may  continue  to  exude 
from  the  clot  for  hours.  It  is  thus  seen  that,  instead  of  the 
plasma  and  corpuscles  of  the  blood  a%it  exists  within  the  living 
body,  coagulation  has  resulted  in  the  formation  of  two  new 
products — serum  and  fibrin — differing  botli  physically  and 
chemically.     These  facts  may  be  put  in  tabular  form  thus: 

Blood  as  it  flows  )  Liquor  sanguinis  (plasma). 
in  tbe  vessels.     (  Corpuscles. 

Blood  after  co-     \  Coagulum  |  £^cles< 
agulation.         j  Serum. 

As  fibrin  may  be  seen  to  arise  in  the  form  of  threads,  under 
the  microscope,  in  coagulating  blood,  and  since  no  trace  of  it  in 
any  form  has  been  detected  in  the  plasma,  and  the  process  can 
be  accounted  for  otherwise,  it  seems  unjustifiable  to  assume  that 
fibrivi  exists  preformed  in  the  blood,  or  arises  in  any  way  prior 
to  actual  coagulation. 

Fibrin  belongs  to  the  class  of  bodies  known  as  proteids,  and 
can  be  distinguished  from  the  other  subdivisions  of  this  group 
of  substances  by  certain  chemical  as  well  as  physical  character- 


THE  BLOOD.  165 

istics.  It  is  insoluble  in  water  and  in  solutions  of  sodium  chlo- 
ride; insoluble  in  hydrochloric  acid,  though  it  swells  in  this 
menstruum. 

It  may  be  whipped  out  from  the  freshly  shed  blood  by  a 
bundle  of  twigs,  wires,  or  other  similar  arrangement  presenting 
a  considerable  extent  of  surface ;  and  when  washed  free  from 
red  blood-cells  presents  itself  as  a  white,  stringy,  tough  sub- 
stance, admirably  adapted  to  retain  anything  entangled  in  its 
meshes.  If  fibrin  does  not  exist  in  the  plasma,  or  does  not  arise 
directly  as  such  in  the  clot,  it  must  have  some  antecedents  al- 
ready existing  as  its  immediate  factors  in  the  plasma,  either 
before  or  after  it  is  shed. 

The  principal  theories  of  coagulation  are  these  :  1.  Coagu- 
lation results  from  the  action  of  a  fibrin-ferment  on  fibrinogen 
and  paraglobulin.  2.  Coagulation  results  from  the  action  of 
a  fibrin-ferment  on  fibrinogen  alone.  Fibrinogen  and  para- 
globulin (see  sections  on  "  The  Chemistry  of  the  Animal  Body") 
are  proteids  originating  from  the  plasma,  during  clotting  in  all 
probability.  Fibrin-ferment  loses  its  properties  on  boiling,  and 
a  very  small  quantity  suffices  in  most  cases  to  induce  the  result. 
For  these  and  other  reasons  this  agent  has  been  classed  among 
bodies  known  as  unorganized  ferments,  which  are  distinguished 
by  the  following  properties  : 

They  exert  their  influence  only  under  well-defined  circum- 
stances, among  which  is' a  certain  narrow  range  of  tempera- 
ture, about  blood-heat  being  most  favorable  for  their  action. 
They  do  not  seem  to  enter  themselves  into  the  resulting  prod- 
uct, but  act  from  without,  as  it  were  (catalytic  action),  hence  a 
very  small  quantity  suffices  to  effect  the  result.  In  all  cases 
they  are  destroyed  by  boiling,  though  they  bear  exposure  for 
a  limited  period  to  a  freezing  temperature. 

From  observations,  microscopic  and  other,  it  has  been  con- 
cluded that  the  corpuscles  play  an  important  part  in  coagula- 
tion by  furnishing  the  fibrin-ferment  ;  but  the  greatest  diver- 
sity of  opinion  prevails  as  to  which  one  of  the  morphological 
elements  of  the  blood  furnishes  the  ferment,  for  each  one  of 
them  has  been  advocated  as  the  exclusive  source  of  this  fer- 
ment bjr  different  observers. 

We  do  not  favor  the  current  theories  of  the  coagulation  of 
the  blood.  We  would  explain  the  whole  matter  somewhat  thus: 
What  the  blood  is  in  chemical  composition  and  other  properties 
from  moment  to  moment  is  the  result  of  the  complicated  inter- 


166  COMPARATIVE  PHYSIOLOGY. 

action  of  all  the  various  cells  and  tissues  of  the  body.  Any  one 
of  these,  departing  from  its  normal  behavior,  at  once  affects  the 
blood  ;  but  health  implies  a  constant  effort  toward  a  certain 
equilibrium,  never  actually  reached  but  always  being  striven 
after  by  the  whole  organism.  The  blood  can  no  more  maintain 
its  vital  equilibrium,  or  exist  as  a  living  tissue  out  of  its  usual 
environment,  than  any  other  tissue.  But  the  exact  circum- 
stances under  which  it  may  become  disorganized,  or  die,  are 
legion  ;  hence,  it  is  not  likely  that  the  blood  always  clots  in 
the  same  way  in  all  groups  of  animals,  or  even  in  the  same 
group.  The  normal  disorganization  or  death  of  the  tissue  re- 
sults in  clotting  ;  but  there  may  be  death  without  clotting,  as 
when  the  blood  is  frozen,  in  various  diseases,  etc. 

To  say  that  fibrin  is  formed  during  coagulation  expresses  in 
a  crude  way  a  certain  fact,  or  rather  the  resultant  of  many 
facts.  To  explain  :  When  gunpowder  and  certain  other  ex- 
plosives are  decomposed,  the  result  is  the  production  of  cer- 
tain gases.  If  we  knew  these  gases  and  their  mode  of  com- 
position but  in  the  vaguest  way,  we  should  be  in  much  the 
same  position  as  we  are  in  regard  to  the  cogulation  of  the 
blood. 

There  is  no  difficulty  in  understanding  why  the  blood  does 
not  clot  in  the  vessels  after  death  so  long  as  they  live,  nor  why 
it  does  coagulate  upon  foreign  bodies  introduced  into  the  blood- 
stream. So  long  as  it  exists  under  the  very  conditions  under 
which  it  began  its  being,  there  is  no  reason  why  the  blood 
should  become  disorganized  (clot).  It  would  be  marvelous  if 
it  did  clot,  for  then  we  could  not  understand  how  it  could  ever 
have  been  developed  as  a  tissue  at  all.  It  is  just  as  reasonable 
to  ask,  Why  does  not  a  muscle-cell  become  rigid  (clot)  in  the 
body  during  life  ? 

Probably  in  no  field  in  physiology  has  so  much  work  been 
done  with  so  little  profit  as  in  the  one  we  are  now  discussing  ; 
and.  as  we  venture  to  think,  owing  to  a  misconception  of  the  real 
nature  of  the  problem.  We  can  understand  the  practical  im- 
portance of  determining  what  circumstances  favor  coagulation 
or  retard  it,  both  within  the  vessels  and  without  them  ;  but 
from  a  theoretical  point  of  view  the  subject  has  been  exalted 
out  of  all  proportion  to  its  importance. 

Coagulation  is  favored  by  gentle  movement,  contact  with 
foreign  bodies,  a  temperature  of  about  38°  to  40°  C,  addi- 
tion of  a  small  quantity  of  water,  free  access  of  oxygen,  etc. 


THE  BLOOD.  167 

The  process  is  retarded  by  a  low  temperature,  addition  of 
abundance  of  neutral  salts,  extract  of  the  mouth  of  the  leech, 
peptone,  much  water,  alkalies,  and  many  other  substances. 
The  excess  of  carbonic  anhydride  and  diminution  of  oxygen 
seem  to  be  the  cause  of  the  slower  coagulation  of  venous  blood, , 
hence  the  blood  long  remains  fluid  in  animals  asphyxiated.  A 
little  reflection  suffices  to  explain  the  action  of  most  of  the  fac- 
tors enumerated.  Any  cause  which  hastens  the  disintegration 
of  the  blood-cells  must  accelerate  coagulation ;  chemical  changes 
underlie  the  changes  in  this  as  in  all  other  cases  of  vital  action. 
Slowing  of  the  blood-stream  to  any  appreciable  extent  likewise 
favors  clotting,  hence  the  explanation  of  the  success  of  the 
treatment  of  aneurisms  by  pressure.  It  is  plain  that  in  all 
such  cases  the  normal  relations  between  the  blood  and  the  tis- 
sues are  disturbed,  and,  when  this  reaches  a  certain  point, 
death  (coagulation)  ensues,  as  with  any  other  tissue. 

Clinical  and  Pathological. — The  changes  in  the  blood  that 
characterize  certain  abnormal  states  are  highly  instructive.  If 
blood  from  an  animal  be  injected  into  the  veins  of  one  of  an- 
other species,  the  death  of  the  latter  often  results,  owing  to  non- 
adaptation  of  the  blood  already  in  the  vessels,  and  to  the  tissues 
of  the  creature  generally.  The  corpuscles  break  up — the  change 
of  conditions  has  been  too  great.  Deficiency  in  the  quantity  of 
the  blood  as  a  whole  (oligcemia)  causes  serious  change  in  the 
functions  of  the  body  ;  but  that  a  haemorrhage  of  considerable 
extent  can  be  so  quickly  recovered  from  speaks  much  for  the 
recuperative  power  of  the  blood-forming  tissues.  Various  kinds 
of  disturbances  in  these  blood-forming  organs  result  in  either 
deficiency  or  excess  of  the  blood-cells,  and  in  some  cases  the 
appearance  of  unusual  forms  of  corpuscles. 

Ancemia  may  arise  from  a  deficiency  either  in  the  numbers 
or  the  quality  of  the  red  cells  ;  they  may  be  too  few,  deficient 
in  size,  or  lacking  in  the  normal  quantity  of  haemoglobin.  In 
one  form  (pernicious  ancemia),  which  often  proves  fatal  in 
man  a  variety  of  forms  in  the  red  blood-cells  may  appear  in  the 
blood-stream ;  some  may  be  very  small,  some  larger  than  usual, 
others  nucleated,  etc.  Again,  the  white  cells  may  be  so  multi- 
plied that  the  blood  may  bear  in  extreme  cases  a  resemblance 
to  milk. 

In  these  cases  there  has  been  found  associated  an  unusual 
condition  of  the  bone-marrow,  the  lymphatic  glands,  the  spleen, 
and,  some  have  thought,  of  other  parts. 


168 


COMPARATIVE  PHYSIOLOGY, 


The  excessive  action  of  these  organs  results  in  the  production 
and  discharge  into  the  blood-current  of  cells  that  are  immature 
and  embryonic  in  character.     This  seems  to  us  an  example  of 


Fig.  153. 


Fig.  149.— Outlines  of  red  corpuscles  in  a  case  of  profound  annemia.  1, 1,  normal  cor- 
puscles; 2,  large  red  corpuscle— megalocyte;  3,  3,  very  irregular  forms— poikilo- 
cyt.es;  4,  very  small,  deep-red  corpuscles— microcytes. 

Fig.  150.— Origin  of  microcytes  from  red  corpuscles  by  process  of  budding  and  fission. 
Specimen  from  red  marrow. 

Fig.  151. — Nucleated  red  blood-corpuscles  from  blood  in  case  of  leukssmia. 

Fig.  152.— Corpuscles  containing  red  blood-corpuscles.  1,  from  blood  of  child  at  term; 
2,  from  blood  of  a  leukemic  patient. 

Fig.  153. — a,  1.  2,  3,  spleen-cells  containing  red  blood-corpuscles,  b,  from  marrow;  1, 
cell  containing  nine  red  corpuscles;  2,  cell  with  reddish  granular  pigment;  3,  fusi- 
form cell  containing  a  single  red  corpuscle,  c,  connective-tissue  corpuscle  from 
subcutaneous  tissue  of  young  rat,  showing  the  intracellular  development  of  red 
blood-corpuscles.    (Figs.  149-153,  after  Osier.) 

a  reversion  to  an  earlier  condition.  It  is  instructive  also  in  that 
the  facts  point  to  a  possible  seat  of  origin  of  the  cells  in  the 
adult,  and,  taken  in  connection  with  otber  facts,  we  may  say,  to 
their  normal  source.  These  blood-producing  organs,  having 
too  much  to  do  in  disease,  do  their  work  badly — it  is  incom- 
plete. 

Althougb  the  evidence,  from  experiment,  to  show  that  the 


THE  BLOOD.  169 

nervous  system  in  mammals,  and  especially  in  man,  has  an  in- 
fluence over  the  formation  and  fate  of  the  blood  generally,  is 
scanty,  there  can  be  little  doubt  that  such  is  the  case,  when  we 
take  into  account  instances  that  frequently  fall  under  the  notice 
of  physicians.  Certain  forms  of  anaemia  have  followed  so  di- 
rectly upon  emotional  shocks,  excessive  mental  work  and  worry, 
as  to  leave  no  uncertainty  of  a  connection  between  these  and 
the  changes  in  the  blood  ;  and  the  former  must,  of  course,  have 
acted  chiefly  if  not  solely  through  the  nervous  system. 

It  will  thus  be  apparent  that  the  facts  of  disease  are  in  har- 
mony with  the  views  we  have  been  enforcing  in  regard  to  the 
blood,  which  we  may  now  briefly  recapitulate. 

Summary. — Blood  may  be  regarded  as  a  tissue,  with  a  fluid 
matrix,  in  which  float  cell-contents.  Like  other  tissues,  it  has 
its  phases  of  development,  including  origin,  maturity,  and 
death.  The  colorless  cells  of  the  blood  may  be  considered  as 
original  imdifferentiated  embryo  cells,  which  retain  their  primi- 
tive character  ;  the  non-nucleated  red  cells  of  the  adult  are  the 
mature  form  of  nucleated  cells  that  in  the  first  instance  are 
colorless,  and  arise  from  a  variety  of  tissues,  and  which  in 
certain  diseases  do  not  mature,  but  remain,  as  they  originally 
were  at  first,  nucleated.  When  the  red  cells  are  no  longer 
fitted  to  discharge  their  functions,  they  are  in  some  instances 
taken  up  by  amoeboid  organisms  (cells)  of  the  spleen,  liver, 
etc. 

The  chief  function  of  the  red  corpuscles  is  to  convey  oxy- 
gen ;  of  the  white,  to  develop  as  required  into  some  more  differ- 
entiated form  of  tissue,  act  as  porters  of  food-material,  and 
probably  to  take  up  the  work  of  many  other  kinds  of  cells 
when  the  needs  of  the  economy  demand  it.  The  fluid  matrix 
or  plasma  furnishes  the  lymph  by  which  the  tissues  are  directly 
nourished,  and  serves  as  a  means  of  transport  for  the  cells  of  the 
blood. 

The  chemical  composition  of  the  blood  is  highly  complex, 
in  accordance  with  the  function  it  discharges  as  the  reservoir 
whence  the  varied  needs  of  the  tissues  are  supplied  ;  and  the 
immediate  receptacle  (together  with  the  lymph)  of  the  entire 
waste  of  the  body  ;  but  the  greater  number  of  substances  exist 
in  very  minute  quantities.  The  blood  must  be  maintained  of 
a  certain  composition,  varying  only  within  narrow  limits,  in 
order  that  neither  the  other  tissues  nor  itself  may  suffer. 

The  normal  disorganization  of  the  blood  results  in  coagula- 


170  COMPARATIVE  PHYSIOLOGY. 

tion,  by  which,  a  substance,  proteid  in  nature,  known  as  fibrin, 
is  formed,  the  antecedents  of  which  are  probably  very  variable 
throughout  the  animal  kingdom,  and  are  likely  so  even  in  the 
same  group  of  animals,  under  different  circumstances  ;  and  a 
substance  abounding  in  proteids  (as  does  also  plasma),  known 
as  serum,  squeezed  from  the  clot  by  the  contracting  fibrin.  It 
represents  the  altered  plasma. 

Certain  well-known  inorganic  salts  enter  into  the  composi- 
tion of  the  blood — both  plasma  and  corpuscles — but  the  princi- 
pal constituent  of  the  red  corpuscles  is  a  pigmented,  ferrugi- 
nous proteid  capable  of  crystallization,  termed  haemoglobin.  It 
is  respiratory  in  function. 


THE   CONTEACTILE  TISSUES. 


That  contractility,  which  is  a  fundamental  property  in  some 
degree  of  all  protoplasm,  becoming'  pronounced  and  definite, 
giving  rise  to  movements  the  character  of  which  can  be  pre- 
dicted with  certainty  once  the  form  of  the  tissue  is  known,  finds 
its  highest  manifestation  in  muscular  tissue. 

Very  briefly,  this  tissue  is  made  up  of  cells  which  may  be 
either  elongated,  fusiform,  nucleated,  finally  striated  lengthwise, 


Fig.  154. 


Fig.  155. 


Fig.  154.—  Muscular  fibers  from  the  urinary  bladder  of  the  human  subject.  1  x  200 
(Sappey.)  1,1,1,  nuclei;  2,2,2.  borders  of  some  of  the  fibers;  3,3,  isolated  fibers; 
4,  4,  two  fibers  joined  together  at  5. 

Fig.  155.— Muscular  fibers  from  the  aorta  of  the  calf.  1  x  200.  (Sappey.)  1, 1,  fibers 
joined  with  each  other;  2,  2,  2,  isolated  fibers. 

but  non-striped  transversely,  united  by  a  homogeneous  cement 
substance,  the  whole  constituting  non-striped  or   involuntary 


172  COMPARATIVE  PHYSIOLOGY. 

muscle  ;  or,  long  nucleated  fibers  transversely  striped,  covered 
with  an  elastic  sheath  of  extreme  thinness,  bound  together 
into  small  bundles  by  a  delicate  connective  tissue,  these  again 
into  larger  ones,  till  what  is  commonly  known  as  a  "  muscle  " 
is  formed.  This,  in  the  higher  vertebrates,  ends  in  tough,  ine- 
lastic extremities  suitable  for  attachment  to  the  level's  it  may  be 
required  to  move  (bones).  Certain  of  the  tissues  will  be  found 
briefly  described  in  the  sections  preceding  "  Locomotion." 

Comparative. — The  lowest  animal  forms  possess  the  power 
of  movement,  which,  as  we  have  seen  in  Amoeba,  is  a  result 
rather  of  a  groping  after  food  ;  and  takes  place  in  a  direction 
it  is  impossible  to  predict,  though  no  doubt  regulated  by  laws 
definite  enough,  if  our  knowledge  were  equal  to  the  task  of  de- 
fining them. 

Those  ciliary  movements  among  the  infusorians,  connected 
with  locomotion  and  the  capture  of  food,  are  examples  of  a 
protoplasmic  rhythm  of  wonderful  beauty  and  simplicity. 

Muscular  tissue  proper  first  appears  in  the  Coelenterata,  but 
not  as  a  wholly  independent  tissue  in  all  cases.  In  many 
ccelente rates  cells  exist,  the  lower  part  of  which  alone  forms  a 
delicate  muscular  fibei',  while  the  superficial  portion  (myoblast), 
composing  the  body  of  the  cell,  may  be  ciliated  and  is  not  con- 
tractile in  any  special  sense.     The  non-striped  muscle-cells  are 

most  abundant  among  the  in- 

..-—"•-••,.,-.  . ..,-•; :"■'■>''  vertebrates,  though  found  in 

,.^/  the  viscera  and  a  few  other 

parts    of    vertebrates.      This 

:  - -J"^  /;'jg_2,_  form  is  plainly  the  simpler 

and    more    primitive.      The 


/  voluntary  muscles  are  of  the 

ri^^r^£SfaSaJelly-fiSh'the^  Griped  variety  in  articulates 

and  some  other  invertebrate 
gi*oups  and  in  all  vertebrates  ;  and  there  seems  to  be  some  re- 
lation between  the  size  of  the  muscle-fiber  and  the  functional 
power  of  the  tissue — the  finer  they  are  and  the  better  supplied 
with  blood,  two  constant  relations,  the  greater  the  contrac- 
tility. 

Whether  a  single  smooth  muscle-cell,  a  striped  fiber  (cell), 
or  a  collection  of  the  latter  (muscle)  be  observed  the  invariable 
result  of  contraction  is  a  change  of  shape  which  is  perfectly 
definite,  the  long  diameter  of  the  cell  or  muscle  becoming 
shorter,  and  the  short  diameter  longer. 


THE  CONTRACTILE  TISSUES. 


173 


Ciliary  Movements.  —  This  subject  has  been  already  con- 
sidered briefly  in  connection  with  some  of  the  lower  forms  of 
life  presented  for  study. 

It  is  to  be  noted  that  there  is  a  gradual  replacement  of  this 
form  of  action  by  that  of  muscle  as  we  ascend  the  animal 
scale  ;  it  is,  however,  retained  even  in  the  highest  animals  in 
the  discharge  of  functions  analogous  to  those  it  fulfills  in  the 
invertebrates. 

Thus,  in  Vorticella,  we  saw  that  the  ciliary  movements  of 
the  peristome  caused  currents  that  carried  in  all  sorts  of  parti- 
cles, including  food.  In  a  creature  so  high  in  the  scale  as  the 
frog  we  find  the  alimentary  tract  ciliated  ;  and  in  man  himself 
a  portion  of  the  respiratory  tract  is  provided  with  ciliated  cells 
concerned  with  assisting  gaseous  interchange,  a  matter  of  the 
highest  importance  to  the  well-being  of  the  mammal.  As  be- 
fore indicated,  ciliated  cells  are  found  in  the  female  generative 
organs,  where  they  play  a  part  already  explained. 

It  is  a  matter  of  no  little  significance  from  an  evolutionary 
point  of  view,  that  cil- 
iated cells  are  more 
widely  distributed  in 
the  foetus  than  in  the 
fully  developed  ani- 
mal. 

As  would  be  ex- 
pected the  movements 
of  cilia  are  affected  by 
a  variety  of  circum- 
stances and  reagents  ; 
thus,  they  are  quick- 
ened by  bile,  acids, 
alkalies,  alcohol,  ele- 
vation of  temperature 
up  to  about  40°  C, 
etc.  ;  retarded  by  cold,  ' 
carbonic  anhydride,  Fk 
ether,  chloroform,  etc. 

In  some  cases  their 
action  may  be  arrested 
and  re-established  by 
treatment  with  rea- 
gents, or  it  may  recommence  without  such  assistance. 


.  157.— Nodes  of  Ranvier  and  lines  of  Fromann 
(Kanvier).  A.  Intercostal  nerve  of  the  mouse, 
treated  with  silver  nitrate.  B.  Nerve-fiber  from 
the  sciatic  nerve  of  a  full-grown  rabbit.  A,  node 
of  Ranvier  ;  J/,  medullary  substance  rendered 
transparent  by  the  action  of  glycerin:  CY,  axis- 
cylinder  presenting  the  lines  of  Fromann,  which 
are  very  distinct  near  the  node.  The  lines  are  less 
marked  at  a  distance  from  the  node. 


All  this 


174 


COMPARATIVE  PHYSIOLOGY. 


seems  to  point  to  ciliary  action  as  falling  under  the  laws  gov- 
erning the  movements  of  protoplasm  in  general.  It  is  impor- 
tant to  bear  in  mind  that  ciliary  action  may  go  on  in  the  cells 
of  a  tissue  completely  isolated  from  the  animal  to  which  it  he- 
longs,  and  though  influenced,  as  just  explained,  by  the  sur- 
roundings, that  the  movement  is  essentially  automatic,  that  is, 
independent  of  any  special  stimulus,  in  which  respect  it  differs 
a  good  deal  from  voluntary  muscle,  which  usually,  if  not  al- 
ways, contracts  only  when  stimulated. 

The  lines  along  which  the  evolution  of  the  contractile  tissues 
has  proceeded  from  the  indefinite  out  do  wings  and  withdrawals 

of  the  substance  of 
Amoeba  up  to  the 
highly  specialized 
movements  of  a 
striped  muscle-cell 
are  not  all  clearly 
marked  out  ;  but 
even  the  few  facts 
mentioned  above 
suffice  to  show  gra- 
dation, intermedi- 
ate forms.  A  sim- 
ilar law  is  involved 


in  the  muscular 
contractility  mani- 
fested by  cells  with 
other  functions. 
The  automatic  (self - 


Fig.  158.— Mode  of  termination  of  the  motor-nerves  (Flint, 
after  Rouget).  A.  Primitive  fasciculus  of  the  thyro- 
hyoid muscle  of  the  human  subject,  and  its  nerve- 
tube  :  1,1,  primitive  muscular  fasciculus;  2,  nerve- 
tube  ;  3,  medullary  substance  of  the  tube,  which  is 
seen  extending  to  the  terminal  plate,  where  it  disap- 
pears ;  4,  terminal  plate  situated  beneath  the  sarco- 
femma— that  is  to  say,  between  it  and  the  elementary 
fibrilhe;  5,  5,  sarcolemma.  B.  Primitive  fasciculus  of 
the  intercostal  muscle  of  the  lizard,  in  which  a  nerve-  Originated,  mcle- 
tube  terminates:  1,1,  sheath  of  the  nerve-tube:  2,  T,0„.i„„+  l.j,,™^!^  ^t 
nucleus  of  the  sheath;  3,  3,  sarcolemma  becoming  peuueiii/  iciigei^  ui 
continuous  with  the  sheath;  4,  medullary  substance  „  cti*miln<A  rhvthm 
of  the  nerve-tube,  ceasing  abruptly  at  the  site  of  the  a  SUmiUUS;  lnymm 
terminal  plate;  5, 5,  terminal  plate;  6.  6,  nuclei  of  the  suefffestive  of  cilia- 
plate;  7,  7.  granular  substance  which  forms  the  princi-  oa 
pal  element  of  the  terminal  plate  and  which  is  con-  ry  movement,  more 
tinuous  with  the  axis  cylinder;   8,  8,  undulations  of    _„Q     •!•--.  i       •  it,, 

the  sarcolemma   reproducing  those  of  the  fibrilhe;    mamiehi      ui       ui t; 
0, 9,  nuclei  of  the  sarcolemma.  earlier       developed 

smooth  muscle  than  in  the  voluntary  striped  muscle  of  higher 
vertebrates,  indicating  further  by  the  regularity  with  which 
certain  organs  act  in  which  this  smooth  muscular  tissue  is  pre- 
dominant, a  relationship  to  ciliary  movement  something  in 
common  as  to  origin — in  a  word,  an  evolution.  And  if  this  be 
borne  in  mind,  we  believe  many  facts  will  appear  in  a  new 


THE  CONTRACTILE  TISSUES. 


175 


light,  and  be  invested  with  a  breadth  of  meaning  they  would 
not  otherwise  possess. 

The  Irritability  of  Muscle  and  Nerve. — An  animal,  as  a  frog, 
deprived  of  its  brain,  will  remain  motionless  till  its  tissues  have 
died,  unless  the  animal  be  in  some  way  stimulated.  If  a  mus- 
cle be  isolated  from  the  body  with  the  nerve  to  which  it  be- 
longs, it  will  also  remain  passive  ;  but,  if  an  electric  current  be 
passed  into  it,  if  it  be  pricked,  pinched,  touched  with  a  hot  body 
or  with  certain  chemical  reagents,  contraction  ensues  ;  the  same 
happening  if  the  nerve  be  thus  treated  instead  of  the  muscle. 
The  changes  in  the  muscle  and  the  nerve  will  be  seen  later  to 
have  much  in  common  ;  the  muscle  alone,  however,  contracts, 
undergoes  a  visible  change  of  form. 


Fig.  159.— Intrafibrillar  terminations  of  the  motor  nerve  in  striated  muscle,  stained 
with  gold  chloride  (Landois). 


Now,  the  agent  causing  this  is  a  stimulus,  and  as  we  have 
seen,  may  be  mechanical,  chemical,  thermal,  electrical,  or  nerv- 
ous. As  both  nerve  and  muscle  are  capable  of  being  function- 
ally affected  by  a  stimulus,  they  are  said  to  be  irritable ;  and 
since  muscle  does  not  contract  without  a  stimulus,  it  is  said  to 
be  non-automatic. 

Now,  since  muscle  is  supplied  with  nerves,  as  well  as  blood- 
vessels, which  end  in  a  peculiar  way  {end  plates)  beneath  the 
muscle-covering  (sarcolemma)  in  the  very  substance  of  the  pro- 
toplasm, it  might  be  that  when  muscle  seemed  to  be  stimu- 
lated, as  above  indicated,  the  responsive  contraction  was  really 
due  to  the  excited  nerve  terminals  ;  and  thus  has  arisen  the 
question,  Is  muscle  of  itself  really  irritable  ? 

What  has  been  said  as  to  the  origin  of  muscular  tissue  points 
very  strongly  to  an  affirmative  answer,  though  it  does  not  fol- 
low that  a  property  once  possessed  in  the  lower  forms  of  a  tissue 
may  not  be  lost  in  the  higher.  From  various  facts  it  may  be 
concluded  that  muscle  possesses  independent  irritability. 


THE   GKAPHTC  METHOD   AND   THE   STUDY  OF 
MUSCLE   PHYSIOLOGY. 


It  is  impossible  to  study  the  physioloay  of  muscle  to  the  best 
advantage  without  the  employment  of  the  graphic  method  ; 

and,  on  the  other  hand,  no 
tissue  is  so  well  adapted  for 
investigation  by  the  isolated 
method — i.  e.,  apart  from  the 
animal  to  which  it  actually 
belongs — as  muscle  ;  hence 
the  convenience  of  introduc- 
ing at  an  early  period  our 
study  of  the  physiology  of 
contractile  tissue  and  illus- 
trations of  the  graphic  meth- 
od, the  general  principles  of 
which  have  already  been 
considered. 

The  descriptions  in  the 
text  will  be  brief,  and  the 
student  is  recommended  to 
examine  the  figures  and  ac- 
companying explanations 
with  some  care. 

Chronographs,  Revolving 
Cylinders,  etc.— Fig.  160  rep- 
resents one  of   the  earliest 

Fin.  100.- -Original  chronometer,  devised  by  „  „        ,-, 

Thomas   young,  for  measuring  minute    forms  ot    apparatus  tor  tUe 
portions  of  time  (after  McKendrick).    a,  ,      e  i     ■   <•  •,+.„ 

cylinder  revolving  on  vertical  axis;  b  measurement  of  brief  mter- 
weight  acting  as  motive  power;  e,d,  small  ,,„ic  nf  ti-me>  pm-isnstiTiP'  of  a 
balls  for  regulating  the  velocity  of  the  vals  or  time>  consisting  01  d 
cylinder;  e,  marker  recording  a  line  on  simple  mechanism  for  pro- 
ducing the  movement  of  a 

cylinder,  which  may  be  covered  with  smoked  paper,  or  other- 


THE  STUDY   OF   MUSCLE   PHYSIOLOGY. 


177 


wise  prepared  to  receive  impressions  made  upon  it  by  a  point 
and  capable  of  being  raised  or  lowered,  and  its  movements  reg- 


C.                                                          ,1 

I. 

1 

J. 

Fig.  161. — Myographic  tracing,  such  as  is  obtained  when  the  cylinder  on  which  it  is 
written  does  not  revolve  during  the  contraction  of  the  muscle  (after  McKendrlck). 

ulated.  The  cylinder  is  ruled  vertically  into  a  certain  number 
of  spaces,  so  that,  if  its  rate  of  revolution  is  known  and  is  con- 
stant (very  important),  the  length  of  time  of  any  event  recorded 
on  the  sensitive  surface  may  be  accurately  known.  This  whole 
apparatus  may  be  considered  a  chronograph  in  a  rough  form. 

But  a  tuning-fork  is  the  most  reliable  form  of  chronograph, 
provided  it  can  be  kept  in  coustant  action  so  long  as  required  ; 


Fig.  162. — Marty's  chronograph  as  applied  to  revolving  cylinder  (after  McKendrick). 
a,  galvanic  element;  b,  wooden  stand  bearing  tuning-fork  (two  hundred  vibrations 
per  second);  c,  electro-magnet  between  limbs  of  tuning-fork:  il.  e.  positions  for 
tuning-forks  of  one  hundred  and  fifty  vibrations  per  second;  /.  tuning-fork  lying 
loose,  which  may  be  applied  to  d\  g,  revolving  cylinder:  h.  electric  chronograph 
kept  in  vibration  synchronous  with  the  tuning-fork  interrupter.  The  current 
working  the  electro-magnet  from  a,  is  interrupted  at  i.  Foucaulfs  regulator  is 
seen  over  the  clock-work  of  the  cylinder,  a  little  to  the  right  of  cj. 

12 


178 


COMPARATIVE   PHYSIOLOGY. 


and  is  provided  with  a  recording  apparatus  that  does  not  cause 
enough  friction  to  interfere  with  its  vibrations. 

Fig.  162  illustrates  one  arrangement  that  answers  these  con- 
ditions fairly  well. 

The  marker,  or  chronograph,  in  the  more  limited  sense,  is 
kept  in  automatic  action  by  the  fork  interrupting  the  current 
from  a  battery  at  a  certain  definite  rate  answering  to  its  own 
proper  note. 

Marey's  chronograph,  which  is  represented  at  h  above,  and 
in  more  detail  below,  in  Fig.  163,  consists  of  two  electro-mag- 
nets armed  with  keepers,  between  which  is  the  writer,  which 


Fio.  163.— Side  view  of  Marey's  chronograph  (after  McKendriek).  a,  a,  coils  of  wire; 
b,  b,  keepers  of  electro-magnets;  c,  vibrating  style  fixed  to  the  steel  plate  e;  d, 
binding  screws  for  attachment  of  wires;  +  from  interrupting  tuning-fork;  —  to 
the  battery. 


has  a  little  mass  of  steel  attached  to  it,  the  whole  working  in 
unison  with  the  tuning-fork,  so  that  an  interruption  of  the  cur- 
rent implies  a  like  change  of  position  of  the  writing-style,  which 
is  always  kept  in  contact  with  the  recording  surface. 

Fig.   173   shows  the  arrangements  for  recording  a  single 

muscle  contraction,  and 
Fig.  174  the  character  of 
the  tracing  obtained. 

A  muscle-nerve  prepa- 
ration, which  usually  con- 
sists of  the  gastrocnemius 
of  the  frog  with  the  sciatic 
nerve  attached,  clamped  by 

Fig.   164.— Muscle-nerve  preparation,  showing  a  portion  of   the  femur  cut 

gastrocnemius  muscle,  sciatic  nerve,  and  „.         ...         ,,                     . 

portion   of    femur  of    frog,  for  attachment  Oil     With       the      muscle,     IS 

to  a  vise  (after  Rosenthal).  made>    Qn    stimulatioilj    to 


THE   STUDY   OF  MUSCLE   PHYSIOLOGY. 


179 


raise  a  weighted  lever  which  is  attached  to  a  point  writing  on  a 
cylinder  moved  by  some  sort  of  clock-work.  In  this  case  the 
cylinder  is  kept  stationary  during  the  contraction  of  the  mus- 
cle ;  hence  the  records  appear  as  straight  vertical  lines. 

For  recording  movements  of  great  rapidity,  so  that  the  in- 
tervals between  them  may  be  apparent,  such  an  apparatus  as  is 


Fig.  165. — Spring  myograph  of  Du  Bois-Rcymond  (after  Rosenthal).  The  arrange- 
ments for  registering  various  details  are  similar  to  those  for  pendulum  myograph 
(Fig.  173). 

figured  here  (Fig.  165)  answers  well,  the  vibrations  of  a  tuning- 
fork  being  written  on  a  blackened  glass  plate,  shot  before  a  chro- 
nograph by  releasing  a  spring. 

Several  records  may  be  made  successively  by  more  compli- 
cated arrangements,  as  will  be  explained  by  another  figure  later. 


THE   APPARATUS   USED   FOR   THE    STIMULATION   OF 
MUSCLE. 

It  is  not  only  important  that  there  should  be  accurate  and 
delicate  methods  of  recording  muscular  contractions,  but  that 
there  be  equally  exact  methods  of  applying,  regulating,  and 
measuring  the  stimulus  that  induces  the  contraction. 

Fig.  166  gives  a  representation  of  the  inductorium  of  Du 
Bois-Reymond,  by  which  either  a  single  brief  stimulation  or 
a  series  of  such  repeated  with  great  regularity  and  frequency 


180 


COMPARATIVE   PHYSIOLOGY. 


Fig.  166. — Du  Bois-Reymond's  inductorium  (after  Rosenthal),  i,  secondary  coil;  c, 
primary  coil;  b,  electro-magnet;  A, armature  of  hammer; /,  small  movable  screw. 
The  current  from  battery,  ascending  metal  pillar,  passes  along  hammer,  and  by 
6crew  gets  into  primary  coil,  thus  inducing  current  in  secondary  coil.  By  con- 
nection between  primary  coil  and  wires  around  soft  iron  of  b,  iron  becomes  a  mag- 
net, hammer  is  attracted  from  screw/,  and  current  thus  broken;  but  when  this 
occurs,  soft  iron  ceases  to  be  a  magnet  necessarily,  and,  hammer  springing  back, 
the  whole  course  of  events  is  repeated.  This  may  occur  several  hundred  times  in 
a  second.  The  above  may  be  clearer  from  diagram,  Fig.  167.  By  sliding  second- 
ary coil  up  and  down,  strength  of  induced  current  can  be  graduated. 


may  be  effected.     The  apparatus  consists  essentially  of  a  pri- 
mary coil,  secondary  coil,  magnetic   interrupter,  and   a   scale 


Fig.  167.— Diagrammatic  representation  of  the  working  of  Fig.  166  (after  Rosenthal) 


THE   STUDY  OF   MUSCLE  PHYSIOLOGY. 


181 


to  determine  the  relative  strength  of  the  current  employed. 
The  instrument  is  put  into  action  by  one  or  more  of  the  various 
well-known  galvanic  cells,  of  which  Daniell's  are  suitable  for 
most  experiments. 


Fig.  169. 


Fig.  168. 


Fig.  168.— Pflfiger's  myograph.  The  muscle  may  be  fixed  to  the  vise  C  in  the  moist 
chamber,  the  vise  connecting  with  the  lever  E E,  the  point  of  which  touches  the 
plate  of  smoked  glass  G.  The  lever  is  held  in  equipoise  by  //.  When  weights  are 
placed  in  scale-pan  F,  the  lever  writes  the  degree  of  extension  effected  (after  Ro- 
senthal). 

Fig.  169. — Tetanizing  key  of  Du  Bois-Iteymond  (after  Rosenthal).  Wires  may  be 
attached  at  b  and  c.  When  d  is  down  the  current  is  "short-circuited,"  i.  e..  does 
not  pass  through  the  wires,  but  direct  from  c  through  d  to  b.  or  the  reverse,  since 
6,  c,  d  are  of  metal,  and,  on  account  of  their  greater  cross-section,  conduct  so 
much  more  readily  than  the  wires,  a  is  an  insulating  plate  of  ebonite.  This  form 
of  key  is  adapted  for  attachment  to  a  table,  etc. 


The  access  to,  or  exclusion  of  the  current  from,  the  indue- 
torium  is  effected  by  some  of  the  forms  of  keys,  a  specimen  of 
which  is  illustrated  in  Fig.  169. 

The  moist  chamber,  or  some  other  means  of  preventing  the 
drying  of  the  preparation,  which  would  soon  result  in  impaired 


182 


COMPARATIVE   PHYSIOLOGY. 


action,  followed  by  death,  is  essential.  A  moist  chamber  con- 
sists essentially  of  an  inclosed  cavity,  in  which  is  placed  some 
wet  blotting-paper,  etc.,  and  is  usually  made  with  glass  sides. 
The  air  in  such  a  chamber  must  remain  saturated  with  moisture. 

A  good  knowledge  of  the  subject  of  electricity  is  especially 
valuable  to  the  student  of  physiology.  But  there  are  a  few  ele- 
mentary facts  it  is  absolutely  necessary  to  bear  in  mind  :  1.  An 
induced  current  exists  only  at  the  moment  of  making  or  break- 
ing a  primary  (battery)  current.  2.  At  the  moment  of  making, 
the  induced  current  is  in  the  opposite  direction  to  that  of  the 
primary  current,  and  the  reverse  at  breaking.  3.  The  strength 
of  the  induced  current  varies  with  the  strength  of  the  primary 
current.  4.  The  more  removed  the  secondary  coil  from  the 
primary  the  weaker  the  current  (induced)  becomes. 

The  clock-work  mechanism  and  its  associated  parts,  as  seen 
in  Fig.  170,  on  the  right,  is  usually  termed  a  myograph. 


Fig.  170.— Arrangement  of  apparatus  for  transmission  of  muscular  movement  by  tam- 
bours (after  McKendrick).  a,  galvanic  element;  b,  primary  coil;  c,  secondary  coil 
of  inductorium;  d,  metronome  for  interrupting  primary  circuit  when  induction 
current  is  sent  to  electrodes  k\  h,  forceps  for  femur;  the  muscle,  which  is  not 
here  represented,  is  attached  to  the  receiving  tambour  g,  by  which  movement  is 
transmitted  to  recording  tambour  e,  which  writes  on  cylinder/. 


Instead  of  muscular  or  other  movements  being  communi- 
cated directly  to  levers,  the  contact  may  be  through  columns 
of  air,  which,  it  will  be  apparent,  must  be  capable  of  communi- 
cating very  slight  changes  if  the  apparatus  responds  readily  to 
the  alterations  in  volume  of  the  inclosed  air. 

Fig.  171  represents  a  Marey's  tambour,  which  consists  essen- 


THE   STUDY   OF   MUSCLE   PHYSIOLOGY. 


183 


Fig.  171. — Tambour  of  Marey  (after  McKendrick).  a,  metallic  case;  b,  thin  India-rub- 
ber membrane;  c,  thin  disk  of  aluminium  supporting  lever  d,  a  small  portion  of 
which  only  is  represented;  e,  screw  for  placing  support  of  lever  vertically  over'c; 
/j  metallic  tube  communicating  with  cavity  of  tambour  for  attachment  to  an  In- 
dia-rubber tube. 


tially  of  a  rigid  metallic  case  provided  with  an  elastic  top,  to 
which  a  lever  is  attached,  the  whole  being  brought  into  com- 
munication with  a  column  of  air  in  an  elastic  tube.  The  work- 
ing of  such  a  mechanism  will  be  evident  from  Figs  170  and  172. 

1 


Fig.  172.— Tambours  of  Marey  arranged  for  transmission  of  movement  (after  McKen- 
drick). a,  receiving  tambour;  b,  India-rubber  tube;  c,  registering  tambour;  d, 
spiral  of  wire,  owing  to  elasticity  of  which,  when  tension  is  removed  from  a,  the 
lever  ascends. 

The  greatest  danger  in  the  use  of  such  apparatus  is  not  fric- 
tion but  oscillation,  so  that  it  is  possible  that  the  original  move- 
ment may  not  be  expressed  alone  or  simply  exaggerated,  but 
also  complicated  by  additions,  for  which  the  apparatus  itself  is 
responsible. 


184 


COMPARATIVE   PHYSIOLOGY. 


Fih.  173. 


THE  STUDY  OF  MUSCLE   PHYSIOLOGY.  185 

Fig.  173.— Diagrammatic  representation  of  the  pendulum  myograph.  The  smoked- 
glass  plate.  A,  swings  with  a  pendulum,  B.  Before  an  experiment  is  commenced 
the  pendulum  is  raised  up  to  the  right  and  kept  in  position  by  the  tooth,  a.  catch- 
ing on  the  spring-catch,  b.  On  depressing  the  catch,  b,  the  glass  plate  being  set 
free  swings  into  the  new  position  indicated  by  the  dotted  lines,  and  is  held  there 
by  the  tooth,  «',  meeting  the  catch,  b' .  In  the  course  of  its  swing  the  tooth,  a, 
coming  into  contact  with  the  projecting  steel  rod,  c,  knocks  it  to  one  side,  into 
the  position  indicated  by  the  dotted  line,  c'.  The  rod,  c,  is  in  electric  continuity 
with  the  wire,  x,  of  the"primary  coil  of  an  induction  machine.  In  like  manner 
the  screw,  d,  is  in  electric  continuity  with  the  wire,  y,  of  the  same  primary  coil. 
The  screw,  d,  and  the  rod,  c.  are  provided  with  platinum  points,  and  both  are  in- 
sulated by  means  of  the  ebonite  block,  e.  The  circuit  of  the  primary  coil  to  which 
.rand  y  belong  is  closed  as  long  as  c  and  d  are  in  contact.  When  in  its  swing 
the  tooth.  «',  knocks  c  away  from  d,  the  circuit  is  immediately  broken,  and  a 
•' breaking"  shock  is  sent  through  the  electrodes  connected  with  the  secondary 
coil  of  the  machine,  and  so  through  the  nerve.  A  lever  is  brought  to  bear  on  the 
glass  plate, and  when  at  rest  describes  an  arc  of  a  circle  of  large  radius.  The  tun- 
ing-fork,/(ends  only  seen),  serves  to  mark  the  time  (after  Foster). 

Apparatus  of  this  kind  is  not  usually  employed  much  for 
experiments  with  muscle  ;  such  an  arrangement  is,  however, 
showm  in  Fig.  170,  in  which  also  will  be  seen  a  metronome,  the 
pendulum  of  wdiich,  by  dipping  into  cups  containing  mercury, 
makes  the  circuit.  Such  or  a  simple  clock  may  be  utilized  for 
indicating  the  longer  intervals  of  time,  as  seconds. 


A   SINGLE    SIMPLE   MUSCULAR   CONTRACTION. 

Experimental  Facts.— The  phases  in  a  single  twitch  or  mus- 
cular contraction  may  be  studied  by  means  of  the  pendulum 
myograph  (Fig.  173).  It  consists  of  a  heavy  pendulum,  which 
swings  from  a  position  on  the  right  to  a  corresponding  one  on 
the  left,  where  it  is  secured  by  a  catch.  During  the  swing  of 
the  pendulum,  which  carries  a  smoked-glass  plate  (by  means 
of  arrangements  more  minutely  described  below  the  figure),  a 
tuning-fork  writes  its  vibrations  on  the  plate,  on  which  is  in- 
scribed the  marking  indicating  the  exact  moment  of  the  break- 
ing of  an  electric  current,  which  gives  rise  to  a  muscle  contrac- 
tion that  is  also  recorded  on  the  plate. 

The  tracing  on  analysis  presents  :  1.  The  record  of  a  tuning- 
fork  making  one  hundred  and  eighty  vibrations  in  a  second. 
2.  The  parallel  marking  of  the  lever  attached  to  the  muscle 
before  it  began  to  rise.  3.  A  curve,  at  first  rising  slowly,  and 
then  rapidly  to  a  maximum.  4.  A  cmwe  of  descent  similar  in 
character,  but  somewhat  more  lengthened. 

We  may  interpret  this  record  somewhat  thus  :  1.  A  rise  of 
the  lever  answering  to  the  shortening  of  the  muscle  to  which  it 
is  attached  following  upon  the  momentary  induction  shock, 
as  the  entrance  of  the  current  into  the  nerve,  the  stimulation  of 
which  causes  the  contraction,  may  be  called.    2.  A  period  before 


186  COMPx\RATIVE   PHYSIOLOGY. 

the  contraction  begins,  which,  as  shown  by  the  time  marking, 

occupies  in  this  case  — -  ,  or  about  TV  of  a  second.    In  the  tracing 

the  upward  curve  indicates  that  the  contraction  is  at  first  rela- 
tively slow,  then  more  rapid,  and  again  slower,  till  a  brief  sta- 


a  b 


Fig.  174.— Muscle-curve  obtained  by  the  pendulum  myograph  (Foster).  Read  from 
left  to  right.  The  latent  period  is  indicated  by  the  space  between  a  and  b,  the 
length  of  which  is  measured  by  the  waves  of  a  tuning-fork,  making  one  hundred 
and  eighty  double  vibrations  in  a  second;  and  in  like  manner  the  duration  of  the 
other  phases  of  the  contraction  may  be  estimated. 

tionary  period  is  reached,  when  the  muscle  gradually  but  rap- 
idly returns  to  its  previous  condition,  passing  through  the  same 
phases  as  during  contraction  proper.  In  other  words,  there  is 
a  period  of  rising  and  of  falling  energy,  or  of  contraction  and 
relaxation.  4.  A  period  during  which  invisible  changes,  as 
will  be  explained  later,  are  going  on,  answering  to  those  in  the 
nerve  that  cause  the  molecular  commotion  in  muscle  which 
precedes  the  visible  contraction — the  latent  period,  or  the  period 
of  latent  stimulation. 

The  facts  may  be  briefly  stated  as  follows  :  The  stimulation 
of  a  muscle  either  directly  or  through  its  nerve  causes  contrac- 
tion, followed  by  relaxation,  both  of  which  are  preceded  by  a 
latent  period,  during  which  no  visible  but  highly  important 
molecular  changes  are  taking  place.  The  whole  change  of  events 
is  of  the  briefest  duration,  and  is  termed  a  muscle  contraction. 
The  tracing  shows  that  the  latent  period  occupied  rather  more 
than  Tfs  second,  the  period  of  contraction  proper  about  T^,  and 
of  relaxation  ^  second,  so  that  the  whole  is  usually  begun  and 
ended  within  TV  second  ;  yet,  as  will  be  learned  later,  many 
chemical  and  electrical  phenomena,  the  concomitants  of  vital 
change,  are  to  be  observed. 

In  the  case  just  considered  it  was  assumed  that  the  muscle 


THE  STUDY   OF  MUSCLE   PHYSIOLOGY. 


1S7 


was  stimulated  through  its  nerve.  Precisely  the  same  results 
would  have  followed  had  the  muscle  been  caused  to  contract  by 
the  momentary  application  of  a  chemical,  thermal,  or  mechanical 
stimulus. 

If  the  length  of  nerve  between  the  point  of  stimulation  and 
the  muscle  was  considerable,  some  difference  would  be  observed 


Fig.  175. — Diagrammatic  representation  of  the  measurement  of  velocity  of  nervous 
impulse  (Foster).  Tracing  taken  by  pendulum  myograph  (Fig.  173).  The  nerve 
of  same  muscle-nerve  preparation  is  stimulated  in  one  case  as  far  as  possible  from 
muscle,  in  the  other  as  near  to  it  as  possible.  Latent  period  is  ab,  ab'.  respect- 
ively. Difference  between  ab  and  ab'  indicates,  of  course,  length  of  time  occu- 
pied by  nervous  impulse  in  traveling  along  nerve  from  distant  to  near  point. 

in  the  latent  period  if  in  a  second  case  the  nerve  were  stimu- 
lated, say,  close  to  the  muscle.  This  is  represented  in  Fig.  175, 
in  which  it  is  seen  that  the  latent  period  in*  the  latter  case  is 
shortened  by  the  distance  from  b'  to  &,  which  must  be  owing 
to  the  time  required  for  those  molecular  changes  which,  occur- 
ring in  a  nerve,  give  rise  to  a  contraction  in  the  muscle  to  which 
it  belongs ;  in  fact,  we  have  in  this  method  the  means  of  estimat- 
ing the  rate  at  which  these  changes  pass  along  the  nerve — in 
other  words  we  have  a  means  of  measimng  the  speed  of  the 
propagation  of  a  nervous  impulse.  The  estimated  rate  is  for  the 
frog  twenty-eight  metres  per  second,  and  for  man  about  thirty- 
three  metres.  As  the  latter  has  been  estimated  for  the  nerve, 
with  its  muscle  in  position  in  the  living  body,  it  must  be  re- 
garded rather  as  a  close  approximation  than  as  exact  as  the 
other  measurements  referred  to  in  this  chapter. 

It  will  be  borne  in  mind  that  the  numbers  given  as  repre- 
senting the  relative  duration  of  the  events  vary  with  the  ani- 
mal, the  kind  of  muscle,  and  a  variety  of  conditions  affecting 
the  same  animal. 


TETANIC    CONTRACTION. 

It  is  well  known  that  a  weight  may  be  held  by  the  out- 
stretched arm   with   apparently   perfect  steadiness  for  a  few 


188 


COMPARATIVE   PHYSIOLOGY. 


seconds,  but  that  presently  the  arm  begins  to  tremble  or  vi- 
brate, and  soon  the  weight  must  be  dropped.  The  arm  was 
maintained  in  its  position  by  the  joint  contraction  of  several  mus- 
cles, the  action  of  which  might  be  described  (traced)  by  a  writer 
attached  to  the  hand  and  recording  on  a  moving  surface.  Such 
a  record  would  indicate  roughly  what  had  happened ;  but  the 
exact  nature  of  a  muscular  contraction  in  such  a  case  can  best  be 
learned  by  laying  bare  a  single  muscle,  say  in  the  thigh  of  a 
frog,  and  arranging  the  experiment  so  that  a  graphic  record 
shall  be  made. 

Using  the  apparatus  previously  described  (Fig.  173),  a  series 
of  induction  shocks  may  be  sent  into  the  muscle  with  the  result 
indicated  in  Figs.  176  and  177,  according  to  the  rate  of  interrup- 
tion of  the  current. 


j* 


1** 


^^w»*''*iw~*1* ' — \ 


Fig.  176.— Curve  of  imperfect,  tetanic  contraction  (Foster).  Uppermost  tracing  indi- 
cates contractions  of  muscle;  intermediate,  when  the  shocks  were  given;  lower, 
time-markings  of  intervals  of  one  second.  Curve  to  be  read,  like  others,  from  left 
to  right,  and  illustrates  at  the  end  a  ."'contraction  remainder." 

If  the  stimuli  follow  each  other  with  a  certain  rapidity,  such 
a  tracing  as  that  represented  in  Fig.  176  is  obtained ;  and  if  the 
rapidity  of  the  stimulation  exceeds  a  fixed  rate,  the  result  is  that 
seen  in  Fig.  177. 


Fro.  li'T.— Curve  of  complete  tetanic  contraction  (Foster). 


THE  STUDY  OP  MUSCLE   PHYSIOLOGY.  189 

It  is  possible  to  see  in  these  tracings  a  genetic  relation,  the 
second  figure  being  evidently  derivable  from  the  first,  and  the 
third  from  the  second,  by  the  fusion  of  all  the  curves  into  one 
straight  line. 

The  Muscle  Tone. — There  are  a  number  of  experimental  facts 
from  which  the  conclusion  has  been  drawn  that  tetanic  contrac- 
tion is  accompanied  by  a  muscle  tone  whioii  is  in  itself  evidence 
of  the  nature  of  the  contraction. 

We  may  safely  conclude  that,  at  all  events,  most  of  the  mus- 
cular contractions  occurring  within  the  living  body  are  tetanic 
— i.  e.,  the  muscle  is  in  a  condition  of  shortening,  with  only  very 
brief  and  slight  phases  of  relaxation ;  and  that  a  comparatively 
small  number  of  individual  contractions  suffice  for  tetanus 
when  caused  by  the  action  of  the  central  nervous  system; 
though,  as  proved  by  experiments  on  muscle  removed  from  the 
body,  they  may  be  enormously  increased.  While  a  few  stimu- 
lations per  second  suffice  to  cause  tetanus,  it  will  also  persist 
though  thousands  be  employed. 

THE  CHANGES  IN  A  MUSCLE  DURING  CONTRACTION. 

Though  the  change  in  form  is  very  great  during  the  con- 
traction of  a  muscle,  the  change  in  bulk  is  almost  inappreci- 
able, amounting  to  a  diminution  of  not  more  than  about  T-gVo 
of  the  volume.  In  fact,  according  to  the  latest  investigator, 
there  is  no  diminution  whatever. 

Since  the  fibers  of  striped  muscle  are  of  very  limited  length 
(30  to  40  mm.),  it  would  seem  that  a  contraction  originating  in 
one  fiber  must  be  capable  of  initiating  a  similar  action  in  its 
neighbor ;  and,  as  the  ends  of  the  fibers  lie  in  contact,  it  is  easy 
to  understand  how  the  wave  of  contraction  spreads.  Normally, 
the  contraction  must  pass  from  about  the  center  of  the  muscle- 
cell  where  the  nerve  terminates  in  the  end-plate. 

THE   ELASTICITY   OF   MUSCLE. 

In  proportion  as  bodies  tend  to  resume  their  original  form 
when  altered  by  mechanical  force  are  they  elastic,  and  the  ex- 
tent to  which  they  do  this  marks  the  limit  of  their  elasticity. 

If  a  muscle  (best  one  with  bundles  of  fibers  of  about  equal 
length  and  parallel  arrangement)  be  stretched  by  a  weight 
attached  to  one  end,  it  will,  on  removal  of  the  extending  force, 


190 


COMPARATIVE   PHYSIOLOGY. 


return  to  its  original  length ;  and  if  a  series  of  weights  which 
differ  by  a  common  increment  be  applied  in  succession  and  the 
degrees  of  extensions  compared,  as  may  be 
done  by  the  graphic  method,  it  will  be  ap- 
parent that  the  increase  in  the  extension 
does  not  exactly  correspond  with  incre- 
mnent  in  the  weight,  but  is  proportionally 
less.  With  an  inorganic  body,  as  a  watch- 
spring,  this  is  not  the  case. 

Further,  the  recoil  of  the  muscle  after 
the  removal  of  the  weight  is  not  perfect 
for  all  weights ;  but  within  certain  narrow 
limits  this  is  the  case,  i.  e.,  the  elasticity 
of  muscle,  though  slight  (for  it  is  easily 
over-extended),  is  perfect.  When  once  a 
muscle  is  over-extended,  so  weighted  that 
it  can  not  reach  its  original  length  almost 
at  once,  it  is  very  slow  to  recover,  which 
explains  the  well-known  duration  of  the 
effects  of  sprains,  no  doubt  owing  to  some 
profound  molecular  change  associated  with 
the  stretching. 

The  tracings  below  show  at  a  glance 
the  difference  between  the  elasticity  of 
muscle  and  of  ordinary  bodies. 

It  is  a  curious  fact  that  a  muscle  during 
the  act  of  contraction  is  more  extensible 
than  when  passive  ;  a  disadvantage  from 
a  purely  physical  point  of  view,  but  prob- 
ably a  real  advantage  as  tending  to  obviate 
tey.   sprain  by  preventing  too  sudden  an  appli- 
mond's   apparatus    for   cation  of  the  extending  force, 
tension  in  muscle  (after         It  will  be  borne  in  mind  that  the  limbs 
SrSd)"  at?ahchldadtUo  are  held  together  as  by  elastic  bands  slight 
mitnCaeien8°  be  observed   ly  on  the  stretch,  owing  to  the  elasticity 
of  the  muscles.      Now,  as  seen  in  many 
tracings  of  muscular  contraction,  there  is  a  tendency  to  imper- 
fect relaxation  after  contraction — the  contraction  remainder 
or  elastic  after-effect,  which  can  be  overcome  by  gentle  trac- 
tion.    In  the  living  body,  the  weight  of  the  limbs  and  the  action 
of  the  stretched  muscles  on  the  side  of  the  limb  opposite  to  that 
on  which  the  muscles  in  actual  contraction  are  situated,  com- 


THE  STUDY   OF   MUSCLE   PHYSIOLOGY.  191 

bine  to  make  the  action  of  the  muscle  more  perfect  by  over- 
coming this  tendency  to  imperfect  relaxation,  which  is  proba- 


Fig.  179.— Illustrations  of  the  difference  in  elasticity  of  inanimate  and  living  matter 
(after  Yeo).  1.  Shows  graphically  behavior  of  a  steel  spring  under  equal  incre- 
ments of  weight.  2.  A  similar  tracing  obtained  from  an  India-rubber  band.  3. 
The  same  from  a  frog's  muscle.  Note  that  the  extension  decreases  with  equal  in- 
crements of  weight,  and  that  the  muscle  fails  to  return  to  the  original  position 
(abscissa)  after  removal  of  the  weight. 

bly  less  marked,  independent  of  these  considerations,  in  the 
living  body.  This  elasticity  of  living  muscles,  which  is  com- 
pletely lost  on  death,  is  a  fair  measure  of  their  state  of  health 
or  organic  perfection.  Hence  that  hard  (elastic  recoil)  feeling 
of  the  muscles  in  young  and  vigorous  persons,  especially  ath- 
letes, in  whom  muscle  is  brought  to  the  highest  degree  of  per- 
fection. 

This  property  is  then  essentially  the  outcome  of  vitality, 
which  is  in  a  word  the  foundation  of  the  differences  noted  be- 
tween the  elasticity  of  inorganic  and  organic  bodies.  A  mus- 
cle, the  nutrition  of  which  is  suffering  from  whatever  cause, 
whether  deficient  blood-supply,  fatigue,  or  actual  disease,  is 
deficient  in  elasticity.  We  wish  to  emphasize  these  relations, 
for  we  consider  it  very  important  to  avoid  regarding  vital  phe- 
nomena in  the  light  of  physics  merely,  which  the  employment 
of  the  graphic  method  (and  indeed  all  methods  by  which  we  re- 
move living  things  out  of  their  normal  relations)  fosters. 

Electrical  Phenomena  of  Mnscle.  —  The  contraction  and 
probably  the  resting  stage  of  muscle  are  attended  by  the  gen- 
eration of  electrical  currents,  the  direction  of  which  is  indicated 
in  Fig.  180. 

It  will  be  observed  that  the  diagram  indicates  that  between 
no  current  and  the  strongest  obtainable  there  are  all  shades  of 


192 


COMPARATIVE   PHYSIOLOGY. 


Fig.  180.— Representation  of  electrical  currents  in  a  muscle-rhombus  (after  Rosenthal). 

strength,  according  to  the  parts  of  the  muscle  connected  by  the 
electrodes.  The  strongest  is  that  resulting  when  the  superfi- 
cial equator  and  the  transverse  center  are  connected  ;  and  it  is 
found  that  the  nearer  these  points  are  approached  the  stronger 
the  current  becomes. 

It  is  important  to  note  that  the  electric  current  of  muscle, 
however  viewed,  is  associated  with  the  chemical  and  all  the 
other  molecular  changes  of  which  the  actual  contraction  is 
but  the  outward  and  visible  sign  ;  and  since  the  currents  have 
an  appreciable  duration,  wane  with  the  vitality  of  the  tissue, 
and  wholly  disappear  at  death,  they  must  be  associated  with  the 
fundamental  facts  of  organic  life  ;  for  it  is  to  be  remembered 
that  electrical  currents  are  not  confined  to  muscle,  but  have 
been  detected  in  the  developing  embryo,  and  even  in  vegetable 
protoplasm.  Though  the  evidence  is  not  yet  complete,  it  seems 
likely  that  electrical  phenomena  may  prove  to  be  associated 
with  (we  designedly  avoid  any  more  definite  expression)  all 
vital  phenomena. 

Chemical  Changes  in  Muscle.— At  a  variable  period  after 
death  the  muscles  become  rigid,  producing  that  stiffness  {rigor 
mortis)  so  characteristic  of  a  recent  cadaver. 


THE  STUDY  OP  MUSCLE  PHYSIOLOGY.  193 

The  subject  can  be  studied  in  some  of  its  aspects  to  great  ad- 
vantage in  an  isolated  individual  muscle. 

Three  changes  in  a  muscle  that  has  passed  into  death  rigor 
are  constant  and  pronounced.  The  living  muscle,  either  alka- 
line or  neutral  in  reaction,  has  become  decidedly  acid  ;  an 
abundance  of  carbonic  anhydride  is  suddenly  given  off  ;  and 
myosin,  a  specific  proteid,  has  been  formed.  That  these  phe- 
nomena have  some  indissoluble  connection  with  each  other  so 
far  as  the  first  two  at  least  are  concerned,  while  not  absolutely 
certain,  seems  probable,  as  will  be  learned  shortly. 

It  will  be  borne  in  mind  that  muscle-fibers  are  tubes  con- 
taining semifluid  protoplasm,  and  that  a  coagulation  of  the  lat- 
ter must  give  rise  to  general  rigor.  This  protoplasmic  substance 
can  be  extracted  at  a  low  temperature  from  the  muscles  of  the 
frog,  and,  as  the  temperature  rises,  coagulates  like  blood,  giving 
rise  to  a  clot  (myosin)  and  muscle-serum,  a  fluid  not  very  unlike 
the  serum  of  blood. 

This  myosin  can  also  be  extracted  from  dead  rigid  muscles 
by  ammonium  chloride,  etc.  It  resembles  the  globulins  gen- 
erally, but  is  less  soluble  in  saline  solutions  than  the  globulin 
of  blood  (paraglobulin)  ;  is  less  tough  than  fibrin  ;  has  a  very 
low  coagulating  point  (55°  to  60°  C.)  ;  and  is  somewhat  jelly- 
like in  appearance.  The  clotting  of  blood  and  of  muscle  is  thus 
analogous,  myosin  answering  to  fibrin,  and  there  being  a  serum 
in  each  case,  both  processes  marking  the  permanent  disorgani- 
zation of  the  tissue.  The  reaction  seems  to  be  due  to  the  forma- 
tion of  a  kind  of  lactic  acid,  probably  sarcolactic  ;  though 
whether  due  to  excessive  production  of  this  acid,  on  the  death 
of  the  muscle,  which  for  some  reason  does  not  remain  free  in 
the  living  muscle,  or  whether  sarcolactic  acid  arises  as  a  new 
product,  is  uncertain.  It  is  certain  that  the  acid  reaction  of 
dead  muscle  is  not  owing  to  carbonic  acid,  for  the  reddened 
litmus  does  not  change  color  on  drying. 

That  a  muscle  in  action  does  use  up  oxygen  and  give  off 
carbonic  anhydride  can  be  definitely  proved  ;  though  it  is 
equally  clear  that  the  life  of  a  muscle  is  not  dependent  on  a 
constant  supply  of  oxygen  as  is  that  of  the  individual,  for  a 
muscle  can  live,  even  contract  long  and  vigorously,  in  an  atmos- 
phere free  from  this  gas,  as  in  nitrogen. 

From  the  suddenness  of  the  increase  of  carbonic  anhydride, 
the  onset  of  death  and  rigor  mortis  has  been  compared  to  an 
explosion. 

13 


194  COMPARATIVE   PHYSIOLOGY. 

After  this  the  muscle  becomes  greatly  changed  physically ; 
its  elasticity  and  translucency  are  lost  ;  there  is  absence  of 
muscle-currents  ;  it  is  wholly  unheritable,  is  less  extensible — it 
is,  as  before  stated,  firmer — it  is  dead. 

But  these  fundamental  phenomena,  the  increase  of  carbonic 
anhydride  and  the  acid  reaction,  are  observable  after  prolonged 
tetanus.  It  was,  therefore — putting  all  the  facts  together  that 
we  now  refer  to  and  others,  not  forgetting  that  a  muscle  is 
always  respiring,  inhaling  oxygen,  and  exhaling  carbonic  an- 
hydride —  not  um^easonable  to  conclude  that  normal  tetanus 
and  rigor  mortis  were  but  exaggerated  conditions  of  a  natural 
state.  The  coagulation  of  the  muscle  protoplasm  {plasma), 
giving  rise  to  myosin,  was,  however,  a  serious  obstacle  to  the 
adoption  of  this  view.  But  it  has  very  recently  been  urged 
with  great  plausibility  that  an  old  view  is  correct,  viz.,  that 
rigor  mortis  (contracture)  is  the  last  act  of  muscle-life  ;  it  is,  in 
fact,  a  prolonged  tetanus  or  contracture,  ending  in  most  cases, 
though  not  all,  in  coagulation  of  the  myosin.  This  state  can 
be  induced  and  recovered  from  in  favorable  cases  by  cutting 
off  the  blood  from  a  part  by  ligature,  and  later  readmitting  it 
to  the  starving  region.  It  has  been  suggested  that  the  prod- 
ucts of  the  muscle-waste,  usually  washed  away  by  the  blood- 
stream, in  such  an  experiment  and  after  death,  collect  and  act 
as  a  stimulant  to  the  muscle,  causing  it  to  remain  in  permanent 
contraction. 

The  other  constituents  of  dead  muscle  and  their  relative 
properties  may  be  learned  from  the  following  table  (Von  Bibra): 

Water 744'5 

Solids :  Myosin,  elastic  substance,  etc.,  in- 
soluble in  water 155*4 

Soluble  proteids 19*3 

Gelatin 207 

Extractives  and  salts 37'1 

Fats 230 

255-5— 255-5 

Total 1,000 

Among  the  extractives  of  muscle  very  important  is  creatin 
(-2  to  '3  per  cent),  a  nitrogenous  crystalline  body.  Certain 
allied  forms,  as  xanthin,  hypoxanthin  (sarkin),  carnin,  taurin, 
and  uric  acid,  are  also  found. 

Glycogen  (animal  starch),  very  abundant  in  all  the  tissues, 


THE  STUDY   OF  MUSCLE    PHYSIOLOGY.  195 

including  the  muscles  of  the  embryo,  is  found  in  small  quantity 
in  the  muscles  of  the  adult;  and  in  the  heart-muscle  a  peculiar 
sugar  (inosit)  is  present. 

It  is,  of  course,  very  difficult  to  say  to  what  extent  the  bodies 
known  as  extractives  exist  in  living  muscle,  though  that  glyco- 
gen, fats,  and  certain  salts  are  normally  present  admits  of  little 
doubt. 

There  is  a  coloring  matter  in  muscle,  more  abundant  in  the 
red  muscles  of  certain  animals  than  the  pale,  allied  to  haemo- 
globin, if  not  identical  with  that  body. 

It  may  be  stated  as  a  fact,  the  exact  significance  of  which 
is  unknown,  that  during  contraction  the  extractives  soluble  in 
water  decrease,  while  those  soluble  in  alcohol  increase. 

It  will,  however,  be  very  plain,  from  what  has  been  stated 
in  this  section,  that  life  processes  and  chemical  changes  are 
closely  associated,  and  to  realize  this  is  worth  much  to  the 
student  of  Nature. 

THERMAL  CHANGES  IN  THE  CONTRACTING  MUSCLE. 

Since  very  marked  chemical  changes  accompany  muscular 
contraction,  it  might  be  expected  that  there  would  be  some 
modification  in  temperature,  and  probably  in  the  direction  of 
elevation.     Experiment  proves  this  to  be  the  case. 

But  why  should  a  muscle  when  at  rest,  as  may  be  shown, 
maintain  a  certain  temperature,  unless  chemical  changes  are 
constantly  taking  place  ?  As  already  stated,  such  is  the  case, 
and  the  rise  on  passing  into  tetanus  is  simply  an  expression  of 
increased  chemical  action. 

No  machine  known  to  us  resembles  muscle  except  super- 
ficially. The  steam-engine  changes  fuel  into  heat  and  mechani- 
cal motion,  but  there  the  resemblance  ends.  Muscle  changes 
its  food,  or  fuel,  not  directly  either  into  heat  or  motion,  but  into 
itself  ;  yet  as  a  machine  it  is  more  effective  than  the  steam- 
engine,  for  more  work  and  less  heat  are  the  outcome  of  its 
activity  than  is  the  case  with  the  steam-engine. 

THE   PHYSIOLOGY    OF   NERVE. 

Muscle  and  nerve  are  constantly  associated  functionally, 
and  have  so  much  in  common  that  it  becomes  desirable  to  study 
them  together.      Much  that  has  been  established  for  muscle 


196 


COMPARATIVE  PHYSIOLOGY. 


holds  equally  well  for  nerve ;  and  the  latter,  though  apparently 
wholly  different  in  structure  at  first  sight,  is  really  not  so. 
Nerve  has  its  protoplasmic  part  (axis-cylinder),  which  is  the 
essential  structure,  its  protective  sheaths,  and  its  nuclei  (nerve- 
corpuscles). 

As  already  indicated,  a  nerve  possesses  irritability. 

It  is  found  that  when  the  constant  (polarizing)  current  is 
passing  from  above  downward  —  that  is,  when  the  cathode 
(negative-pole)  is  on  the  side  toward  the  muscle — the  irritability 
of  the  nerve  is  increased,  and  the  reverse  when  the  opposite 
conditions  prevail. 

This  altered  condition  is  known  as  electrotonus. 

It  has  been  found  as  the  result  of  many  experiments  that 
profound  modifications  of  the  irritability  of  a  nerve  do  take 
place  during  the  passage  of  a  constant  current.  These  are 
diagrammatically  represented  in  Fig.  181. 


Fig.  181. — Diagrammatic  representation  of  variations  in  electrotonus  according  to 
strength  of  current  employed  (after  Prliiger).  nn',  a  section  of  nerve;  a,  anode 
(+  pole);  k,  cathode  (—pole).  Curves  above  the  horizontal  denote  cateleetroto- 
nus;  below,  the  opposite. 

Briefly  stated,  they  are  these  :  1.  The  nature  of  the  change 
depends  on  the  direction  of  the  polarizing  (constant)  current  ; 
hence,  if  the  current  is  descending,  there  is  an  increase  of  irri- 
tability (catelectrotonus)  in  the  portion  of  the  nerve  nearest  the 
muscle,  and  vice  versa.  2.  The  extent  of  the  change  of  irrita- 
bility is  dependent  on  the  strength  of  the  polarizing  current. 
.  3.  This  change  is  most  marked  close  to  the  electrodes,  spreads 
to  a  considerable  extent  beyond  this  point  without  the  elec- 
trodes (extra-polar  regions),  and  also  exists  within  the  region 
of  contact  of  the  electrodes  (intra-po]ar  regions).  4.  It  follows 
that  there  must  be  a  point  at  which  it  is  not  experienced  (indif- 
ferent point  or  neutral  point). 

Now,  it  is  possible  to  understand  why  a  sudden  change  in 


THE  STUDY   OF   MUSCLE  PHYSIOLOGY.  197 

the  current  should  cause  a  muscular  contraction.  An  equally 
sudden  alteration,  a  profound  molecular  effect,  has  been  caused, 
and  this  we  must  believe  essential  to  the  causation  of  a  muscu- 
lar contraction  through  the  influence  of  a  nerve. 

To  use  an  illustration  which  may  serve  a  good  purpose  if 
not  taken  too  literally,  it  is  a  well-known  experience  that  one 
sitting  in  a  room  in  which  a  clock  is  ticking  soon  fails  to  no- 
tice this  regular  sound  :  but  should  the  clock  stop  suddenly  or 
as  suddenly  commence  to  tick  very  rapidly,  the  attention  is 
aroused,  while  a  very  gradual  slowing  to  cessation  or  the  re- 
verse would  have  escaped  notice.  The  explanation  of  such 
facts  takes  us  down  to  the  very  foundations  of  biology  ;  but 
just  now  we  wish  only  to  elucidate  by  our  own  experience 
how  it  is  possible  to  conceive  of  a  muscle  being  stimulated 
by  the  molecular  movements  of  nerve,  or  rather  a  change  in 
these. 

There  are  important  practical  aspects  to  this  question.  One 
may  understand  why  it  is  that  electricity  proves  so  ready  a 
stimulus,  and  is  so  valuable  a  therapeutic  agent.  It  seems,  in 
fact,  as  will  be  learned  later,  to  be  capable  of  taking  the  place 
to  some  extent  of  that  constant  nerve  influence  which  we  be- 
lieve is  being  exerted  in  the  higher  animals  toward  the  mainte- 
nance of  the  regularity  of  their  cell-life  (metabolism). 

Pathological  and  Clinical.— It  is  believed  that  in  the  nerves 
of  a  living  animal  body,  the  electrotonic  condition  can  be  in- 
duced as  in  an  isolated  piece  of  nerve.  Hence,  the  value  of 
the  constant  current  in  diminishing  nerve  irritability  in  neu- 
ralgia and  allied  conditions.  Apparatus  of  great  nicety  of  con- 
struction and  capable  of  generating,  accurately  measuring,  and 
conveniently  applying  electrical  currents  of  different  kinds,  now 
adds  to  the  resources  of  the  practitioner.  But  we  are  probably 
as  yet  only  on  the  threshold  of  electro-therapeutics. 

Electrical  Organs. — Electrical  properties  can  be  manifested 
by  a  large  number  of  fishes ;  and  the  subject  is  of  special  theo- 
retical interest.  It  is  now  established  that  the  development  of 
electrical  organs  points  to  their  being  specially  modified  mus- 
cles— tissues,  in  fact,  in  which  the  contractile  substance  has  disap- 
peared and  the  nervous  elements  become  predominant  and 
peculiar.  No  work  is  done,  but  the  whole  of  the  chemical 
energy  is  represented  by  electi*icity.  Functionally  an  electric 
organ  (which  usually  is  some  form  of  cell,  on  the  wTalls  of 
which  nerves  are  distributed,  inclosing  a  gelatinous  substance, 


198 


COMPARATIVE   PHYSIOLOGY. 


the  whole  being  very  suggestive  of  a  galvanic  battery)  closely 
resembles  a  muscle-nerve  preparation  or  its  equivalent  in  the 

normal  body.  The  electric 
organs  experience  fatigue  ; 
have  a  latent  period  ;  their  dis- 
charge is  tetanic  (interrupted) ; 
is  excited  by  mechanical,  ther- 
mal, or  electrical  stimuli  ;  and 
the  effectiveness  of  the  organs 
is  heightened  by  elevation  of 
temperature,  and  the  reverse 
by  cooling,  etc. 


MUSCULAR    WORK. 


V 


% 


^ 


m 


Fig.  182.— Tlie  electric-fish  torpedo,  dissect- 
ed to  show  electric  apparatus  (Huxley). 
b,  branchiae;  c,  brain;  e,  electric  organ; 
ff,  cranium;  me,  spinal  cord;  //,,  nerves 
to  pectoral  fins  ;  nl,  nervi  laterales  ; 
np,  branches  of  pneumpgastric  nerves 
to  electric  organs;  o,  eye. 


If  during  a  given  period 
one  of  two  persons  raises  a 
weight  through  the  same 
height  but  twice  as  frequent- 
ly as  the  other,  it  is  plain  that 
he  does  twice  the  work  ;  from 
such  a  case  we  may  deduce  the 
rule  for  calculating  work,  viz., 
to  multiply  the  weight  and 
height  together. 

The  effectiveness  of  a  given 
muscle  must,  of  course,  depend 
on  the  degree  to  which  it  shortens,  which  is  from  one  half  to 
three  fifths  of  its  length;  and  the  number  of  fibers  it  contains 
— i.  e.,  upon  its  length  and  the  area  of  its  cross-section,  taking 
into  account  in  connection  with  the  first  factor  the  arrangement 
of  the  fibers ;  those  muscles  in  which  the  fibers  run  longitudinal- 
ly being  capable  of  the  greatest  total  shortening. 

There  is,  as  shown  by  actual  experimental  trial,  a  relation 
between  the  work  done  and  the  load  to  be  lifted.  With  double 
the  weight  tbe  contraction  may  be  as  great  as  at  first,  or  even 
greater  ;  but  a  limit  is  soon  reached  beyond  which  contraction 
is  impossible.  This  principle  may  be  stated  thus:  The  contrac- 
tion is  a  function  of  the  stimulus,  and  is  illustrated  by  the 
diagram  below  (Fig.  183). 

It  has  been  shown  experimentally  that  the  chemical  inter- 
changes in  a  muscle,  acting  against  a  considerable  resistance, 


THE  STUDY   OF   MUSCLE   PHYSIOLOGY. 


199 


are  increased — i.  e.,  the  metabolism  and  the  working  tension 
are  related. 

These  experimental  facts  harmonize  with  our  experience  of 
a  sense  of  satisfaction  and  effectiveness  in  the  use  of  the  muscles 


"i — r 


T~l-T 


10 


£0       30       40 


45 


50       55 


60 


65       70 


80       90      100 


Fig.  183.— Diagram  of  muscular  contractions  with  same  stimulus  and  increasing 
weights.    The  numbers  represent  grammes  (McKendrick). 

when  weights  are  held  in  the  hands  ;  and  it  must  be  a  matter 
of  practical  importance  that  each  person  should,  in  taking  sys- 
tematic exercise,  keep  to  that  kind  which  does  not  either  over- 
weight or  underweight  the  muscles. 


CIRCUMSTANCES   INFLUENCING   THE    CHARACTER 
OF   MUSCULAR   AND   NERVOUS   ACTIVITY. 

The  Influence  of  Blood-Supply.  Fatigue.— Fig.  184  shows  at 
a  glance  differences  in  the  curves  made  by  a  contracting  muscle 
suffering  from  increasing  fatigue. 


ISO  D  V. 


Fig.  184.— Curves  of  a  muscle  contraction  in  different  stages  of  fatigue  (after  Yeo). 
A,  curve  when  muscle  was  fresh:  B,  C,  D,  E,  each  just  after  muscle  had  already- 
contracted  two  hundred  times.  The  alteration  iu  length  of  latent  period  is  not 
well  brought  out  in  these  tracings. 

Suppose  that  in  such  a  case  the  blood  had  been  withheld 
from  the  muscle,  and  that  it  is  now  admitted,  an  almost  im- 
madiate  effect  is  seen  in  the  nature  of  the  contractions  ;  but 
even  if  only  saline  solution  had  been  sent  through  the  vessels  of 
the  muscle,  a  similar  change  would  have  been  noticeable.  We 
may  fairly  conclude  that  the  blood  and  saline  removed  some- 
thing which  had  been  exercising  a  depressing  effect  on  the 
vitality  of  the  muscle.  In  a  working  muscle,  like  all  living 
tissues,  there  are  products  of  vital  action  (metabolism)  that  are 
poisonous.  We  have  already  learned  tbat  a  working  muscle 
generates  an  excess  of  carbonic  anhydride,  and  something  which 
gives  it  an  acid  reaction  ;  and  that  it  uses  up  oxygen  as  well  as 
other  matters  derivable  from  the  blood. 


200  COMPARATIVE  PHYSIOLOGY. 

Fatigue  will  occur,  it  is  well  known,  if  the  muscles  are  used 
for  an  indefinitely  long  period,  no  matter  how  favorable  the 
blood-supply— another  evidence  that  there  is,  in  all  probability, 
some  chemical  product,  the  result  of  their  own  activity,  depress- 
ing them;  and  this  is  rendered  all  the  more  likely  when  it  is 
learned  that  the  injection  of  lactic  acid,  to  take  one  example, 
produces  effects  like  ordinary  fatigue. 

It  is  also  a  matter  of  common  experience  that  exercise,  while 
beneficial  to  the  whole  body,  the  muscles  included,  as  shown  by 
their  enlargement  under  it,  becomes  injurious  when  carried  to 
the  point  of  fatigue. 

Why  the  use  of  the  muscles  is  conducive  to  their  welfare  is 
but  a  part  of  a  larger  question,  Why  does  the  use  of  any  tissue 
improve  it  ? 

When  the  nerve  which  supplies  a  muscle  is  stimulated  its 
blood-vessels  dilate,  and  it  has  been  assumed  that  the  same 
happens  when  a  muscle  contracts  normally  in  the  body ;  and 
when  muscular  action  is  increased  there  is  a  corresponding 
augmentation  in  the  quantity  of  blood  driven  through  the 
muscles  in  a  given  period,  even  if  there  be  no  actual  increase 
in  the  caliber  of  the  blood-vessels,  for  the  heart-beat  is  greatly 
accelerated. 

But  repose  is  as  necessary  as  exercise  for  the  greatest  effect- 
iveness of  the  muscles,  as  the  experience  of  all,  and  especially 
athletes,  proves. 

That  the  nervous  system  plays  a  great  part  in  the  nutrition 
of  muscles  is  evident  from  the  fact,  among  countless  others, 
that  it  is  not  possible  to  use  the  brain  to  its  greatest  capacity 
and  the  muscles  to  their  fullest  at  the  same  time ;  the  individual 
engaged  in  physical  "  training  "  must  forego  severe  mental  ap- 
plication. Nervous  energy  is  required  for  the  muscles,  and  all 
questions  of  blood-supply  are,  though  important,  subordinate. 
But  it  would  be  premature  to  enter  into  a  full  discussion  of  this 
interesting  topic  now. 

The  sense  of  fatigue  experienced  after  prolonged  muscular 
action  is  complex,  though  there  can  be  no  doubt  that  the  nerve- 
centers  must  be  taken  into  account,  since  any  muscular  work 
that,  from  being  unusual,  requires  closer  attention  and  a  more 
direct  influence  of  the  will,  is  well  known  to  be  more  fatigu- 
ing. On  the  other  hand,  the  accumulation  of  products  of 
fatigue  doubtless  reports  itself  through  the  local  nervous  mech- 
anism. 


THE  STUDY   OF   MUSCLE   PHYSIOLOGY.  201 

Separation  of  Muscle  from  the  Central  Nervous  System.— 
When  the  nerve  belonging  to  a  muscle  is  divided,  certain  his- 
tological changes  ensue,  which  may  be  briefly  described  as 
fatty  degeneration,  followed  by  absorption ;  and  when  regener- 
ation of  the  nerve-fibers  takes  place  on  apposition  of  the  cut 
ends,  a  more  or  less  complete  restoration  of  the  functions  of 
the  nerve  follows,  but  the  exact  nature  of  the  process  of  repair 
is  not  yet  fully  agreed  upon ;  it  seems,  in  fact,  to  vary  in  differ- 
ent cases  as  to  details,  though  it  is  likely  that,  in  instances  in 
which  there  is  a  complete  return  to  the  normal  functionally, 
the  axis-cylinders,  at  all  events,  are  reproduced. 

The  degeneration  downward  is  complete  ;  upward,  only  to 
the  first  node  of  Ranvier. 

Immediately  after  the  section  the  irritability  of  the  nerve  is 
increased,  but  rapidly  disappears,  from  the  center  toward  the 
periphery  (Ritter-Valli  law). 

In  the  mean  time  the  muscle  has  been  suffering.  Its  nota- 
bility at  first  diminishes,  then  becomes  greater  than  usual  to 
shocks  from  the  make  or  break  of  the  constant  current  ;  but 
finally  all  irritability  is  lost,  and  fatty  degeneration  and  disap- 
pearance of  true  muscular  structure  complete  the  history.  It 
is  theoretically  interesting,  as  well  as  of  practical  importance, 
that  degeneration  may  be  delayed  by  the  use  of  the  constant 
current,  the  significance  of  which  we  have  already  endeavored 
to  explain. 

The  Influence  of  Temperature,— If  a  decapitated  frog  be 
placed  in  water  of  the  ordinary  temperature,  and  heat  be 
gradually  applied,  the  animal  does  not  move  (proving  that  the 
spinal  cord  alone  is  not  conscious),  but  the  muscles,  when  43° 
to  50°  C.  is  reached,  contract  and  become  rigid,  a  condition 
known  as  "  heat-rigor.1' 

There  are  some  advantages  in  investigating  changes  in  tem- 
perature by  the  graphic  method.  Curves  from  a  muscle-nerve 
preparation  show  that  elevation  of  temperature  shortens  the 
latent  period  and  the  curve  of  contraction.  Lowering  the  tem- 
perature has  an  exactly  opposite  effect,  as  might  be  supposed, 
and  these  changes  take  place  in  the  muscles  of  both  cold- 
blooded and  warm-blooded  animals,  though  more  marked  in 
the  latter. 

The  modifications  evident  to  the  eye  are  accompanied  by 
others,  chemical  in  nature,  and  a  comparison  of  these  shows 
that    the    rapidity   and    force    of    the    muscular    contraction 


202  COMPARATIVE  PHYSIOLOGY. 

run  parallel  with  the  rapidity   and   extent  of  the  chemical 
changes. 

Certain  drugs  also  modify  the  form  of  the  muscle-curve  very 
greatly,  so  that  it  appears  that  the'  molecular  action  which  un- 
derlies all  the  phenomena  of  muscle  and  nerve  (for  what  has 
been  said  of  muscle  applies  also  to  nerve,  if  we  substitute  nerv- 
ous impulse  for  contraction)  can  go  on  only  within  those  nar- 
row bounds  wbich,  one  realizes  more  and  more  in  the  study  of 
physiology,  are  set  to  the  activities  of  living  things. 

UNSTRIPED   MUSCLE. 

This  form  of  muscular  tissue  is  characterized  by  its  long 
latent  period,  its  slow  wave  of  contraction,  and  the  prog- 
ress of  the  contraction  being  in  either  a  transverse  or  longi- 
tudinal direction,  a  wave  of  contraction  in  one  cell  being  cap- 
able of  setting  up  a  corresponding  wave  in  adjoining  cells 
even  when  no  nerve-fibers  are  distributed  to  them.  It  is  ex- 
cited, though  less  readily,  by  all  the  kinds  of  stimuli  that  act 
upon  striped  muscle.  In  the  higher  groups  of  animals  this 
tissue  is  chiefly  confined  to  the  viscera  of  the  chest  and  abdo- 
men, constituting  in  the  case  of  some  of  them  the  greater  part 
of  the  whole  organ. 

The  slow  but  powerful  and  rhythmical  contraction  of  this 
form  of  muscle  adapts  it  well  to  the  part  such  organs  play  in 
the  economy.  There  are  variations,  however,  in  the  rapidity, 
force,  regularity,  and  other  qualities  of  the  contraction  in  dif- 
ferent parts ;  thus,  it  is  comparatively  rapid  in  the  iris,  and  ex- 
tremely powerful  and  regular  in  the  uterus,  serving  to  produce 
that  prolonged  yet '  intermittent  pressm^e  essential  under  the 
circumstances  (expulsion  of  the  foetus). 

Comparative. — Muscular  contraction  is  relatively  sluggish 
and  prolonged  among  the  invertebrates,  to  which,  however,  the 
movement  of  the  wings  of  insects  is  a  marked  exception,  some 
of  them  having  been  shown  by  the  graphic  method  to  vibrate 
some  hundreds  of  times  in  a  second. 

The  slow  movements  of  the  snail  are  proverbial.  As  a  rule, 
the  strength  of  the  muscles  of  the  invertebrates  is  incomparably 
greater  than  that  of  vertebrates,  as  witness  the  powerful  grasp 
of  a  crab's  claw  or  a  beetle's  jaws. 

These  facts  are  in  harmony  with  the  generally  slow  metab- 
olism of  most  invertebrates  and  the  lower  vertebrates. 


THE  STUDY  OF  MUSCLE  PHYSIOLOGY.  203 

The  muscles  of  the  tortoise  contract  tardily  but  with  great 
power,  resist  fatigue  well,  retain  their  vitality  under  unfavor- 
able conditions,  and  after  death  for  a  very  long  period  (days). 

Without  resorting  to  elaborate  experiments,  the  student  may 
convince  himself  of  the  truth  of  most  of  the  above  statements 
by  observing  the  movements  of  a  water-snail  attached  to  a  glass 
vessel ;  the  note  made  by  the  buzzing  of  an  insect,  and  compar- 
ing it  with  one  approaching  it  in  pitch  sounded  by  some  instru- 
ment of  music ;  the  force  necessary  to  withdraw  the  foot  or  tail 
of  a  tortoise  ;  the  peristaltic  movements  of  the  intestine  and 
other  organs  in  a  freshly  killed  animal  ;  or  the  action  of  a  bee, 
wasp,  or  wood-boring  beetle  on  the  cork  of  a  bottle  in  which 
one  of  them  may  be  inclosed. 
• 

SPECIAL   CONSIDERATIONS. 

In  the  case  of  weakly  tuberculous  animals  a  sharp  tap  on 
the  chest  will  often  produce  a  contraction  of  the  muscles  thus 
stimulated;  but,  in  addition,  a  local  contraction  lasting  some 
little  time,  known  as  a  tvheal  or  iclio-muscular  contraction,  fol- 
lows. This  phenomenon  seems  to  be  the  result  of  a  special 
irritability  in  such  muscles. 

Cramp  may  arise  under  a  great  variety  of  circumstances, 
but  it  seems  to  be  in  all  cases  either  a  complete  prolonged  teta- 
nus, in  which  there  is  unusual  muscular  shortening  in  severe 
cases,  at  least,  or  the  persistence  of  a  contraction  remainder. 

The  great  differences  known  to  exist  between  individuals  of 
the  same  species  in  strength,  endurance,  fleetness,  and  other 
particulars  in  which  the  muscles  are  concerned,  raise  numer- 
ous interesting  inquiries.  The  build  of  the  greyhound  or  race- 
horse suggests  in  itself  part  of  the  explanation  on  mechanical 
principles,  lung  capacity,  etc.  But  when  it  is  found  that  one 
dog,  horse,  deer,  or  man  excels  another  of  the  same  race  in 
swiftness  or  endurance,  and  there  is  nothing  in  the  form  to 
furnish  a  solution,  we  are  prompted  to  ask  whether  the  muscles 
may  not  contract  more  energetically,  experience  a  shortening 
of  the  latent  period  or  other  phase  of  contraction ;  or  whether 
they  produce  less  of  waste-products  or  get  rid  of  them  more 
rapidly.  The  whole  subject  is  extremely  complicated,  and  we 
may  say  here  that  there  is  some  evidence  to  show  that  in  races 
of  dogs  and  other  animals  which  surpass  their  fellows  the 
nerve  regulating  the  heart  and  lungs  (vagus)  has  greater  power ; 


204  COMPARATIVE  PHYSIOLOGY. 

but,  leaving1  this  and  much  more  out  of  the  account,  it  is  likely 
there  are  individual  differences  in  the  functional  nature  of  the 
muscle.  Of  equal  or  more  importance  is  the  energizing-  influ- 
ence of  the  nervous  system,  which  probably  under  great  excite- 
ment (public  boat-races,  etc.)  acts  to  produce  in  man  those 
supermaximal  contractions  which  seem  to  leave  the  muscle 
long  the  worse  of  its  unusual  action.  The  nerve-centers,  it  is 
likely,  suffer  still  more  from  excessive  discharge  of  nerve-force 
(as  we  may  speak  of  it  for  the  present)  necessary  to  originate 
the  muscular  work.  Hence  the  importance  of  training  in  all 
animals  to  minimize  the  non-effective  expenditure,  ascertain 
the  capacity  possessed,  learn  the  direction  in  which  weaknesses 
lie ;  and  equally  important  the  much  neglected-period  of  rest 
before  actual  contests — if  such  are  to  be  undertaken  at  ail- 
so  that  all  the  activities  of  the  body  may  gather  head,  and  thus 
be  prepared  to  meet  the  unusual  demand  upon  them. 

The  law  of  rhythm  in  organic  nature  is  beautifully  illus- 
trated by  the  behavior  of  nerve  and  especially  muscle;  at  least 
it  is  more  obvious  in  the  case  of  muscle,  at  this  stage  of  our 
progress. 

The  regularity  with  which  one  phase  succeeds  another  in  a 
single  contraction ;  the  essentially  rhythmic  (vibratory)  char- 
acter of  tetanus,  fatigue  and  recovery  ;  the  recurrence  of  in- 
crease and  decrease  in  the  muscle  and  nerve  currents — in  fact, 
the  whole  history  of  muscle  is  an  admirable  commentary  on 
the  truth  of  the  law  of  rhythm,  into  which  in  further  detail 
space  will  not  permit  us  to  enter. 

It  is  a  remarkable  fact  that  the  endurance  of  man,  especially 
civilized  man,  seems  to  be  greater  than  that  of  any  other  mam- 
mal. It  may  be  hazardous  to  express  a  dogmatic  opinion  as  to 
the  reason  of  this,  but  the  influence  of  the  mind  over  the  body 
is  unquestionably  greater  in  man  than  in  any  other  animal  ; 
and,  if  we  are  correct  in  assigning  so  much  importance  to  the 
influence  of  the  nervous  system  in  maintaining  the  proper 
molecular  balance  which  is  at  the  foundation  of  the  highest 
good  of  an  organism,  we  certainly  think  that  it  is  in  this  direc- 
tion we  must  look  for  the  explanation  of  the  above-mentioned 
fact,  and  much  more  that  would  otherwise  be  obscure  in  man's 
functional  life. 

Functional  Variations.— We  have  endeavored,  in  treating 
this  subject  of  muscle,  to  point  out  how  the  phenomena  vary 
with  the  animal,  the  kind  of  muscle,  and   the  circumstances 


THE   STUDY   OF  MUSCLE   PHYSIOLOGY.  205 

under  which  they  are  manifested.  It  may  be  shown  that  every 
one  of  the  qualities  which  a  muscle  possesses  varies  with  the 
temperature,  the  blood-supply,  the  duration  of  its  action,  the 
character  of  the  stimulus,  and  other  modifying  agents.  Not  only 
are  there  great  variations  for  different  groups  of  animals,  but 
lesser  ones  for  individuals  ;  though  the  latter  are  made  more 
evident  indirectly  than  when  tested  by  the  usual  laboratory 
methods  ;  but  they  must  be  taken  account  of  if  we  would  un- 
derstand animals  as  they  are.  Some  of  these  will  be  referred 
to  later. 

If  a  muscle-cell  be  regarded  in  the  aspect  that  we  are  now 
emphasizing,  its  study  will  tend  to  impress  those  fundamental 
biological  laws,  the  comprehension  of  which  is  of  more  impor- 
tance than  the  acquisition  of  any  number  of  facts,  winch,  how- 
ever interesting,  can,  when  isolated,  profit  little. 

Summary  of  the  Physiology  of  Muscle  and  Nerve.— The 
movements  of  a  muscle  are  distinguished  from  those  of  other 
forms  of  protoplasm  by  their  marked  definiteness  and  limit- 
ation. 

The  contraction  of  a  muscle-fiber  (cell)  results  in  an  increase 
in  its  short  transverse  diameter,  and  a  diminution  of  its  long 
diameter,  without  appreciable  change  in  its  total  bulk. 

Muscle  and  nerve  are  not  automatic,  but  are  irritable. 
Though  muscle  normally  receives  its  stimulus  through  a  nerve, 
it  possesses  independent  irritability. 

Stimuli  may  be  mechanical,  chemical,  thermal,  electrical,  and 
in  the  case  of  muscle,  nervous  ;  and  to  be  effective  they  must 
be  applied  suddenly  and  last  for  a  brief  but  appreciable  time. 

Electrical  stimulation,  especially,  is  only  effective  when 
there  is  a  sudden  change  in  the  force  or  direction  of  the  cur- 
rents.    This  applies  to  both  muscle  and  nerve. 

A  muscular  contraction  consists  of  three  phases  :  the  latent 
period,  the  period  of  rising,  and  the  period  of  falling  energy,  or 
of  contraction  and  relaxation. 

When  the  phase  of  relaxation  is  minimal  and  that  of  con- 
traction approaches  continuity,  a  tetanus  results.  The  contrac- 
tions of  the  muscles  in  situ  are  tetanic,  and  are  accompanied 
by  a  low  sound,  evidence  in  itself  of  their  vibratory  character. 

The  prolonged  contraction  of  a  muscle  leads  to  fatigue  ; 
owing  in  part,  at  least,  to  the  accumulation  of  waste-products 
within  the  muscle  which  depress  its  energies. 

This  is  a  necessary  consequence  of  the  fact  that  all  proto- 


206  COMPARATIVE  PHYSIOLOGY. 

plasmic  activity  is  accompanied  by  chemical  change,  and  that 
some  of  these  processes  result  in  the  formation  of  products 
which  are  hurtful  and  are  usually  rapidly  expelled. 

Muscular  contraction  is  accompanied  by  chemical  changes, 
in  which  the  formation  of  carbon  dioxide,  and  some  substance 
that  causes  an  acid  reaction  to  take  the  place  of  an  alkaline  or 
neutral  one.  Since  free  oxygen  is  not  required  for  the  act  of 
contraction,  but  is  still  used  up  by  a  contracting  muscle,  it  may 
be  assumed  that  the  oxygen  that  plays  a  part  in  actual  contrac- 
tion is  intra-molecular. 

Chemical  changes  are  inseparable  from  the  vital  processes 
of  all  protoplasm,  and  the  phenomena  of  muscle  show  that 
they  are  constantly  in  operation,  but  exalted  during  ordinary 
contraction  and  that  tetanic  condition  which  precedes  and 
may  end  in  coagidation  of  muscle  plasma  and  the  formation  of 
myosin.  The  latter  is  a  result  of  the  disorganization  of  muscle, 
and  has  points  of  resemblance  to  the  coagulation  of  the  blood. 

The  contraction  of  a  muscle  and  the  passage  of  a  nervous 
impulse  are  accompanied  by  electrical  changes.  Whether  cur- 
rents exist  in  uninjured  muscle  and  nerve  is  a  matter  of  contro- 
versy. Ml  physiologists  agree  that  they  exist  in  muscle  (and 
nerve)  duiing  functional  activity. 

During  the  passage  of  a  constant  (polarizing)  current  from 
a  battery  through  a  nerve,  it  undergoes  a  change  in  its  irrita- 
bility and  shows  a  variation  in  the  electro-motive  force  of  the 
ordinary  nerve-current  (electrotonus).  This  fact  is  of  thera- 
peutic importance.  The  electrical  phenomena  of  nerve  are 
altogether  more  prominent  than  the  chemical,  the  reverse  of 
which  is  true  of  muscle.  The  activity  of  a  muscle  (and  nerve 
probably)  is  accompanied  by  the  generation  of  heat,  an  exalta- 
tion of  which  takes  place  during  muscular  contraction. 

Rigor  mortis  causes  an  increase  in  temperature  and  the 
chemical  interchanges  which  accompany  the  other  phenomena. 
A  muscle  may  also  become  rigid  by  passing  into  rigor  caloris. 
Living  muscle  is  translucent,  alkaline  or  neutral  in  reaction, 
and  elastic  ;  dead  muscle,  opaque,  acid  in  reaction,  and  devoid 
of  elasticity,  but  firmer  than  living  muscle,  owing  to  coagula- 
tion of  the  muscle-plasma.  Dead  nerve  undergoes  similar 
changes. 

The  elasticity  of  muscle  is  restricted  but  perfect  within  its 
own  limits.  It  differs  from  that  of  inorganic  bodies  in  that  the 
increments  of  extension  are  not  directly  proportional  to  the 


THE  STUDY  OF   MUSCLE  PHYSIOLOGY.  207 

increments  of  the  weight.  When  overstretched,  muscle  does 
not  return  to  its  original  length  (loss  of  elasticity),  hence  the 
serious  nature  of  sprains. 

It  is  important  to  regard  muscular  elasticity  as  an  expression 
of  vital  properties. 

The  work  done  by  a  muscle  is  ascertained  by  multiplying 
the  load  lifted  by  the  height;  and  the  capacity  of  an  individual 
muscle  will  vary  with  its  length,  the  arrangement  of  its  fibers, 
and  the  area  of  its  cross-section  (i.  e.,  the  number  of  fibers). 

The  work  done  may  be  regarded  as  a  function  of  the  resist- 
ance (load),  as  the  contraction  is  also  a  function  of  the  stimulus. 
The  sepai'ation  of  a  muscle  from  its  nerve  by  section  of  the  lat- 
ter leads  to  certain  changes,  most  rapid  in  the  nerve,  which 
show  that  the  two  are  so  related  that  prolonged  independent 
vitality  of  the  muscle  is  impossible,  and  make  it  highly  proba- 
ble that  muscle  is  constantly  receiving  some  beneficial  stimulus 
from  nerve,  which  is  exalted  and  manifest  when  contraction 
takes  place. 

The  study  of  the  development  of  the  electrical  cells  of  cer- 
tain fishes  shows  that  they  are  greatly  modified  muscles  in 
which  contractility,  etc.,  has  been  exchanged  for  a  very  decided 
exaltation  of  electrical  properties.  It  is  likely,  though  not 
demonstrated,  that  all  forms  of  protoplasm  undergo  electrical 
changes — that  these,  in  fact,  like  chemical  phenomena,  are  vital 
constants. 

The  phases  of  the  contraction  of  smooth  muscular  tissue  are 
all  of  longer  duration;  the  contraction- wave  passes  in  different 
directions,  and  may  spread  into  cells  devoid  of  nerves,  which 
we  think  not  unlikely  also  to  be  the  case,  though  less  so,  for  all 
forms  of  muscle. 

The  smooth  muscle-cell  must  be  regarded  as  a  more  primi- 
tive, less  specialized,  form  of  tissue.  Variations  in  all  the  phe- 
nomena of  muscle  with  the  animal  and  the  circumstances  are 
clear  and  impressive.  Finally,  muscle  illustrates  an  evolution 
of  structure  and  function,  and  the  law  of  rhythm. 


THE  NERVOUS   SYSTEM.— GENERAL    CONSIDER- 
ATIONS. 


Since  in  the  higher  vertebrates  the  nervous  system  is  domi- 
nant, regulating  apparently  every  process  in  the  organism,  it 
will  be  well  before  proceeding  further  to  treat  of  some  of  its 
functions  in  a  general  way  to  a  greater  extent  than  we  have  yet 
done. 

Manifestly,  it  must  be  highly  important  that  an  animal  shall 
be  able  to  place  itself  so  in  relation  to  its  surroundings  that  it 
may  adapt  itself  to  them.  Prominent  among  these  adaptations 
are  certain  movements  by  which  food  is  secured  and  dangers 
avoided.  The  movements  having  a  central  origin,  a  peripheral 
mechanism  of  some  kind  must  exist  so  as  to  place  the  centers 
in  connection  with  the  outer  world.  Passing  by  the  evolution 
of  the  nervous  system  for  the  present,  it  is  found  that  in  verte- 
brates generally  there  is  externally  a  modification  of  the  epi- 
thelial covering  of  the  body  {end-organ)  in  which  a  nerve  ter- 
minates, which  latter  may  be  traced  to  a  cell  or  cells  removed 
from  the  surface  (center),  and  from  which  in  most  cases  other 
nerves  proceed. 

The  nervous  system,  we  may  remind  the  student,  consists  in 
vertebrates  of  centers  in  which  nerve-cells  abound,  united  by 
nerve-fibers  and  by  the  most  delicate  form  of  connective  tissue 
known,  in  connection  with  which  there  are  incased  strands 
of  protoplasm  or  nerves  as  outgrowths.  The  main  centers  are, 
of  course,  aggregated  in  the  brain  and  spinal  cord. 

It  is  possible  to  conceive  of  the  work  of  a  nervous  system 
carried  on  by  a  single  cell  and  an  afferent  and  efferent  nerve ; 
but  inasmuch  as  such  an  arrangement  would  imply  that  the 
central  cell  should  act  the  part  of  both  receiving  and  origi- 
nating impulses  (except  it  were  a  mere  conductor,  in  which  case 
there  would  be  no  advantage  whatever  in  the  existence  of  a  cell 
at  all),  according  to  tbe  principle  of  the  physiological  division 


NERVOUS  SYSTEM.— GENERAL   CONSIDERATIONS.  209 

of  labor,  we  might  expect  that  there  would  be  at  least  two  cen- 
tral cells — one  to  receive  and  the  other  to  transmit  impulses — 
or  at  least  that  there  should  be  some  specialization  among  the 
central  cells  ;  and  we  shall  have  good  reason  later  to  believe 
that  this  has  reached  a  surprising  degree  in  the  highest  ani- 
mals. 

Moreover,  it  would  be  a  great  advantage  if  the  termination 
of  the  ingoing  (afferent)  nerve  should  not  lie  exposed  on  the 
surface,  but  be  protected  by  some  form  of  ce)l  that  had  also  the 
power  to  transmit  to  it  the  impressions  received  from  without, 
in  a  form  suitable  to  the  nature  of  the  nerve  and  the  needs  of 
the  organism. 

So  that  a  complete  mechanism  in  its  simplest  form  would 
furnish :  1.  A  periphei'al  cell  or  nerve  end-organ.  2.  An  affer- 
ent or  sensory  nerve.  3.  Two  or  more  central  cells.  4.  An 
efferent  nerve,  usually  connected  with — 5.  A  muscle  or  other 
form  of  cell,  the  action  of  which  may  be  modified  by  the  out- 
going nerve,  or,  as  we  should  prefer  to  say,  by  the  central 
nervous  cells  through  the  efferent  nerve.  The  advantages  of 
the  principal  cells  being  within  and  protected  are  obvious. 

When,  then,  an  impression  made  on  the  peripheral  cell  is 
carried  inward,  there  modified,  and  results  in  an  outgoing  nerv- 
ous impulse  answering  to  the  afferent  one,  giving  rise  to  a  mus- 
cular contraction  or  other  effect  not  confined  to  the  recipient 
cells,  the  process  is  termed  reflex  action. 

Tbe  great  size,  the  multiplicity  of  forms,  the  distinct  out- 
line and  large  nuclei  of  nerve-cells,  suggest  the  probability  that 
they  play  a  very  important  part,  and  such  is  found  to  be  the 
case.  Indeed,  in  some  sense  the  rest  of  the  nervous  system  may 
be  said  to  exist  for  them. 

Probably  nerve-cells  do  sometimes  act  as  mere  conductors 
of  nervous  impulses  originating  elsewhere,  but  such  is  their 
lowest  function.  Accordingly,  it  is  found  that  the  nature  of  any 
reflex  action  depends  most  of  all  on  the  behavior  of  the  central 
cells. 

It  can  not  be  too  well  borne  in  mind  that  nerves  are  con- 
ductors and  such  only.     They  never  originate  impulses. 

The  properties  considered  in  the  last  chapter  are  common  to 
all  kinds  of  nerves  known;  and  though  we  must  conceive  that 
there  are  some  differences  in  the  form  of  impulses,  these  are  to 
be  traced,  not  to  the  nerve  primarily,  but  to  the  organ  in  which 
it  ends  peripherally  or  to  the  central  cells. 
14 


210  COMPARATIVE  PHYSIOLOGY. 

To  return  to  reflex  action,  it  is  found  that  the  muscular  re- 
sponse to  a  peripheral  irritation  vai'ies  with  the  point  stimu- 
lated, the  intensity  of  the  stimulus,  etc.,  but  is,  above  all,  deter- 
mined by  the  central  cells. 

Nerve  influence  may  be  considered  as  following  lines  of 
least  resistance,  and  there  is  much  evidence  to  show  that  an  im- 
pulse having  once  taken  a  certain  path,  it  is  easier  for  it  to  pass 
in  this  direction  a  second  time,  so  that  we  have  the  foundation 
of  the  laws  of  habit  and  a  host  of  interesting  phenomena  in 
this  simple  principle. 

It  is  found  that,  in  a  frog  deprived  of  its  brain  and  sus- 
pended by  the  under  jaw,  there  is  no  movement  unless  some 
stimulus  be  applied  ;  but  if  this  be  done  under  suitable  condi- 
tions, instructive  results  follow,  which  we  now  proceed  to  indi- 
cate briefly.  The  experiments  are  of  a  simple  character,  which 
any  student  may  carry  out  for  himself. 

Experimental. — Preparing  a  frog  by  cutting  off  the  whole 
of  the  upper  jaw  and  brain-case  after  momentary  anaesthesia, 
suspend  the  animal  by  the  lower  jaw  and  wait  till  it  is  perfectly 
quiet.  Add  to  water  in  a  beaker  sulphuric  acid  till  it  tastes 
distinctly  but  not  strongly  sour,  to  be  used  as  a  stimulus.  1. 
Apply  a  small  piece  of  bibulous  paper,  moistened  with  the  acid, 
to  the  inner  part  of  the  thigh  of  the  animal.  The  leg  will  be 
drawn  up  and  the  paper  probably  removed.  Remove  the  paper 
and  cleanse  the  spot.  2.  Apply  a  similar  piece  of  paper  to  the 
middle  of  the  abdomen  ;  one  or  both  legs  will  probably  be 
drawn  up,  and  wipe  off  the  offending  body.  3.  Let  the  foot  of 
the  frog  hang  in  the  liquid ;  after  a  few  moments  it  will  be 
withdrawn.  4.  Repeat,  holding  the  leg  ;  probably  the  other 
leg  will  be  drawn  up.  5.  Apply  stronger  acid  to  the  inside  of 
the  right  thigh ;  the  whole  frog  may  be  convulsed,  or  the  left 
leg  may  be  put  in  action  after  the  right.  Even  if  the  stimulat- 
ing paper  be  applied  near  the  anus,  it  will  be  removed  by  the 
hind-legs.  6.  Beneath  the  skin  Df  the  back,  (posterior  lymph- 
sac)  inject  a  few  drops  of  liquor  strychnia?  of  the  pharama- 
copocia;  after  a  few  minutes  apply  the  same  sort  of  stimulus  to 
the  thigh  as  before.  The  effects  follow  more  quickly  and  are 
much  more  marked-- the  animal,  it  may  be,  passing  into  a  gen- 
eral tetanic  spasm. 

These  experiments  may  be  varied,  but  suffice  to  establish  the 
following  conclusions  :  1.  The  stimulus  is  not  immediately 
effective,  but  requires  to  act  for  a  certain  variable  period,  de- 


NERVOUS  SYSTEM.— GENERAL  CONSIDERATIONS.  211 


6ENSORY  CENTRE 


INHIBITORY  CENTRE 


C-^V  SENSORY  CELL  AND 
AFFERENT  NERVE 


Fig.  187. — Diagram  intended  to  illustrate  nervous  mechanism  of— 1.  automatism;  2, 
reflex  action;  and  3.  how  nervous  impulses  in  the  latter  case  may  pass  into  the 
higher  parts  of  brain  and  become  part  of  consciousness,  or  be  wholly  inhibited. 
A  reflex  or  automatic  center  may,  for  the  sake  of  simplicity,  be  reduced  to  a  sin- 
gle cell,  as  above  on  the  left. 


pending1  chiefly  on  the  condition  of  the  central  nervous  sys- 
tem. 2.  The  movements  of  the  muscles  harmonize  (are  co-ordi- 
nated), and  tend  to  accomplish  some  end — are  purposive.  If 
the  nerve  alone  and  not  the  skin  be  stimulated,  there  may  be  a 
spasm  only  and  not  adaptive  movement.  3.  Nervous  impulses, 
when  very  abundant,  niay  pass  along  unaccustomed  or  less  ac- 
customed paths  (experiments  4  and  5).  This  is  sometimes  spoken 
of  as  the  radiation  of  nervous  impulses. 

The  sixth  experiment  is  very  important,  for  it  shows  that 
the  result  varies  far  more  with  the  condition  of  the  nervous 
centers  (cells)  than  the  stimulus,  the  part  excited,  or  any  other 
factor. 

Automatism. — But,  seeing  that  these  central  cells  have  such 
independence  and  controlling  power,  the  question  arises.  Are 


212  COMPARATIVE   PHYSIOLOGY. 

these,  or  are  there  any  such  cells,  capable  of  originating  im- 
pulses in  nerves  wholly  independent  of  any  stimulus  from 
without  ?  In  other  words,  have  the  nerve-centers  any  true 
automatism  ?  Apparently  this  quality  is  manifested  by  uni- 
cellular organisms  of  the  rank  of  Amceba.  Has  it  been  lost,  or 
has  it  become  a  special  characteristic  developed  to  a  high  degree 
in  nerve-cells  ? 

We  shall  present  the  facts  and  the  opinions  based  on  them 
as  held  by  the  majority  of  physiologists,  reserving  our  own 
criticisms  for  another  occasion  :  1.  The  medulla  oblongata  is 
supposed  to  be  the  seat  of  numerous  small  groups  of  cells,  to  a 
large  extent  independent  of  each  other,  that  are  constantly 
sending  out  nervous  impulses  which,  proceeding  to  certain  sets 
of  muscles,  maintain  them  in  rhythmical  action.  One  of  the 
best  known  of  these  centers  is  the  respiratory.  2.  The  poste- 
rior lymph  hearts  of  the  frog  are  supplied  by  nerves  (tenth 
pair),  which  are  connected,  of  course,  with  the  spinal  cord. 
When  these  nerves  are  cut,  the  hearts  for  a  time  cease  to  beat, 
but  later  resume  their  action.  3.  The  heart  beats  after  all  its 
nerves  are  cut,  and  it  is  removed  from  the  body,  for  many  hours, 
in  cold-blooded  animals.  4.  The  contractions  of  the  intestine 
take  place  in  the  absence  of  food,  and  in  an  isolated  piece  of 
the  gut.  The  intestine,  it  will  be  remembered,  is  abundantly 
supplied  with  nerve-elements.  5.  In  a  portion  of  the  ureters, 
from  which  it  is  believed  nerve-cells  are  absent,  rhythmical  ac- 
tion takes  place. 

Conclusions. — 1.  Whether  the  action  of  the  respiratory  and 
similar  centers  could  continue  in  the  absence  of  all  stimuli  can 
not  be  considered  as  determined.  2.  That  there  are  regular 
rhythmical  discharges  from  the  spinal  nerve-cells  along  the 
nerves  to  the  lymph  hearts  seems  also  doubtfbl.  3.  Later  in- 
vestigations render  the  automaticity  of  the  heart  more  uncer- 
tain than  ever,  so  that  the  result  stated  above  (3)  must  not  be 
interpreted  too  rigidly. 

Similar  doubts  hang  about  the  other  cases  of  apparent  au- 
tomatism. 

As  regards  the  various  comparatively  isolated  collections  of 
cells  known  as  ganglia,  the  evidence,  so  far  as  it  goes,  is  against 
their  possessing  either  automatic  or  reflex  action  ;  and  new 
views  of  their  nature  will  be  presented  in  due  course. 

Nervous  Inhibition. — If  the  pneumogastric  nerve  passing 
from  the  medulla  to  the  heart  of  vertebrates  be  divided  and  the 


NERVOUS  SYSTEM.— GENERAL   CONSIDERATIONS.  213 

lower  (peripheral)  end  stimulated,  a  decided  change  in  the  ac- 
tion of  the  heart  follows,  which  may  be  in  the  direction  of" 
weakening-  or  slowing,  or  positive  arrest  of  its  action. 

Assuming,  for  the  present,  that  the  cells  (center)  of  the  me- 
dulla have  the  power  to  bring  about  the  same  result,  it  is  seen 
that  such  nervous  influence  is  preventive  or  inhibitory  of  the 
normal  cardiac  beat,  so  that  the  vagus  is  termed  an  inhibitory 
nerve.  Such  inhibition  plays  a  very  important  part  in  the 
economy  of  the  higher  animals,  as  will  become  rnore  and  more 
evident  as  we  proceed.  The  nature  of  the  influences  that  pro- 
duce such  remarkable  results  will  be  discussed  when  we  treat 
of  the  heart. 

An  illustration  will  probably  serve  in  the  mean  time  to  make 
the  meaning  of  what  has  been  presented  in  this  chapter  more 
clear  and  readily  grasped. 

In  the  management  of  railroads  a  very  great  variety  of  com- 
plicated results  are  brought  about,  owing  to  system  and  orderly 
arrangement,  by  which  the  wishes  of  the  chief  manager  are 
carried  out. 

Telegraphing  is  of  necessity  extensively  employed.  Sup- 
pose a  message  to  be  conveyed  from  one  office  to  another,  this 
may  (1)  simply  pass  through  an  intermediate  office,  without 
special  cognizance  from  the  operator  in  charge  ;  (2)  the  operator 
may  receive  and  transmit  it  unaltered  ;  (3)  he  may  be  required 
to  send  a  message  that  shall  vary  from  the  one  he  receives  in  a 
greater  or  less  degree  ;  or  (4)  he  may  arrest  the  command  alto- 
gether, owing  to  the  facts  which  he  alone  knows  and  upon 
which  he  is  empowered  always  to  act  according  to  his  best  dis- 
cretion. 

In  the  first  instance,  we  have  an  analogy  with  the  passage 
of  a  nervous  impulse  through  central  fibers,  or,  at  all  events, 
unaffected  by  cells  ;  in  the  second,  the  resemblance  is  to  cells 
acting  as  conductors  merely ;  in  the  third,  to  the  usual  behavior 
of  the  cells  in  reflex  action;  and,  in  the  fourth,  we  have  an  in- 
stance of  inhibition.  The  latter  may  also  be  rendered  clear  by 
the  case  of  a  horse  and  its  rider.  The  horse  is  controlled  by  the 
rider,  who  may  be  compared  to  the  center,  through  the  reins 
answering  to  the  nerves,  though  it  is  not  possible  for  either  rider 
or  reins  to  originate  the  movements  of  the  animal,  except  as 
they 'may  be  stimuli,  which  latter  are  only  effective  when  there 
are  suitable  conditions— when,  in  fact,  the  subject  is  irritable  in 
the  physiological  sense. 


THE   CIRCULATION   OF  THE   BLOOD. 


Every  tissue,  every  cell,  requiring  constant  nourishment, 
some  means  must  necessarily  have  been  provided  for  the  con- 
veyance of  the  blood  to  all  parts  of  the  organism.  We  now 
enter  upon  the  consideration  of  the  mechanisms  by  which  this 
is  accomplished  and  the  method  of  their  regulation. 

Let  us  consider  possible  mechanisms,  and  then  inquire  into 
their  defects  and  the  extent  to  which  they  are  found  embodied 
in  nature. 

That  there  must  be  a  central  pump  of  some  kind  is  evident. 
Assume  that  it  is  one-chambered,  and  with  an  outflow-pipe 
which  is  continued  to  form  an  inflow-pipe.  This  might  be  pro- 
vided with  valves  at  the  openings,  by  which  energy  would  be 
saved  by  the  prevention  of  regurgitation.  In  such  a  system 
things  must  go  from  bad  to  worse,  as  the  tissues,  by  constantly 
using  up  the  prepared  material  of  the  blood,  and  adding  to  it 
their  waste  products,  would  effect  their  own  gradual  starvation 
and  poisoning. 

It  might  be  conceived,  however,  that  waste  at  all  events  was 
got  rid  of  by  the  blood  being  conducted  through  some  elimi- 
nating organs  ;  and  assume  that  one  such  at  least  is  set  aside 
for  respiratory  work.  If  the  blood  in  its  course  anywhere 
passed  through  such  organs,  the  end  would  be  attained  in  some 
degree  ;  but  if  the  division  of  labor  were  considerable,  we 
should  suppose  that,  gaseous  interchange  being  so  very  impor- 
tant as  we  bave  been  led  to  see  from  the  study  of  the  chapters 
on  general  biology,  and  on  muscle,  organs  to  accomplish  this 
work  might  receive  the  blood  in  due  course  and  return  it  to  the 
central  pump  in  a  condition  eminently  fit  from  a  respiratory 
point  of  view. 

Such,  however,  would  necessarily  be  associated  with  a  more 
complicated  pump  ;  and,  if  this  were  so  constructed  as  to  pre- 
vent the  mixture  of  blood  of  different  degrees  of  functional 
value,  higher  ends  would  be  attained. 


THE   CIRCULATION  OP  THE   BLOOD.  215 

Turning  to  the  channels  themselves  in  which  the  blood 
flows,  a  little  consideration  will  convince  one  that  rigid  tubes 
are  wholly  unfit  for  the  purpose.  Somewhere  in  the  course  of 
the  circulation  the  blood  must  flow  sufficiently  slowly,  and 
through  vessels  thin  enough  to  permit  of  that  interchange  be- 
tween the  blood  and  the  tissues,  through  the  medium  of  the 
lymph,  which  is  essential  from  every  point  of  view.  The  main 
vessels  must  have  a  strength  sufficient  to  resist  the  force  with 
which  the  blood  is  driven  into  them. 

Now,  it  is  possible  to  conceive  of  this  being  accomplished 
with  an  intermittent  flow  ;  but  manifestly  it  would  be  a  great 
advantage,  from  a  nutritive  aspect,  that  the  flow  and  therefore 
the  supply  of  tissue  pabulum  be  constant.  With  a  pump  regu- 
larly intermittent  in  action,  provided  with  valves,  elastic  tubes 
having  a  resistance  in  them  somewhere  sufficient  to  keep  them 
constantly  over-distended,  and  a  collection  of  small  vessels  with 
walls  of  extreme  thinness,  in  which  the  blood-current  is  greatly 
slackened,  a  steady  blood-flow  would  be  maintained,  as  the 
student  may  readily  convince  himself,  by  a  few  experiments  of 
a  very  simple  kind  : 

1.  To  show  the  difference  between  rigid  tubes  and  elastic 
ones,  let  a  piece  of  glass-rod,  drawn  out  at  one  end  to  a  small 
diameter,  have  attached  to  the  other  end  a  Higginson's  (two- 
bulb)  syringe,  communicating  with  a  vessel  containing  water. 
Every  time  the  bulb  is  squeezed,  water  flows  from  the  end  of 
the  glass  rod,  but  the  outflow  is  perfectly  intermittent. 

2.  On  the  other  hand,  with  a  long  elastic  tube  of  India-rub- 
ber, ending  in  a  piece  of  glass  rod  drawn  out  to  a  point  as  be- 
fore, if  the  action  of  the  pump  (bulb)  be  rapid  the  outflow  will 
be  continuous.  An  apparatus  that  every  practitioner  of  medi- 
cine requires  to  use  answers  perhaps  still  better  to  illustrate 
these  and  other  principles  of  the  circulation,  such  as  the  pulse, 
the  influence  of  the  force  and  frequency  of  the  heart-beat  on  the 
blood-pressure,  etc.  We  refer  to  a  two-bulb  atomizer,  the  bulb 
nearer  the  outflow  serving  to  maintain  a  constant  air-pressure. 

We  may  now  examine  the  most  perfect  form  of  heart 
known,  that  of  the  mammal,  in  order  to  ascertain  how  far  it 
and  its  adjunct  tubes  answer  to  a  priori  expectations. 

The  Mammalian  Heart.— In  order  that  the  student  may  gain 
a  correct  and  thorough  knowledge  of  the  anatomy  of  the  heart 
and  the, workings  of  its  various  parts,  we  recommend  him  to 
pursue  some  such  course  as  the  following  : 


216 


COMPARATIVE   PHYSIOLOGY. 


1.  To  consult  a  number  of  plates,  such  as  are  usually  fur- 
nished in  works  on  anatomy,  in  order  to  ascertain  in  a  general 
way  the  relations  of  the  heart  to  other  organs,  and  to  the  chest 
wall,  as  well  as  to  become  familiar  with  its  own  structure. 

2.  To  supplement 
this  with  reading  the 
anatomical  descrip- 
tions, without  too  great 
attention  to  details  at 
first,  but  with  the  ob- 
ject of  getting  his  ideas 
clear  so  far  as  they  go. 

3.  Then,  with  plates 
and  descriptions  before 
him,  to  examine  sever- 
al dead  specimens  of 
the  heart  of  the  sheep, 
ox,  pig,  or  other  mam- 
mal, first  somewhat 
generally,  then  syste- 
matically, with  the 
purpose  of  getting  a 
more  exact  knowledge 
of  the  various  struct- 
ures and  their  anatom- 
ical as  well  as  physi- 
ological relations. 

We  would  not  have 
the  student  confine  his 
attention  to  any  single 
form  of  heart,  for  each 
shows  some  one  struct- 
ure better  than  the 
others  ;  and  the  addi- 
tional advantages  of 
comparison  are  very 
great.  The  heart  of 
the  ox,  from  its  size, 
is  excellent  for  the  study  of  valvular  action,  and  the  framework 
with  which  the  muscles,  valves,  and  vessels  are  connected  ; 
while  the  heart  of  the  pig  (and  dog)  resemble  the  human  organ 
more  closely  than  most  others  that  can  be  obtained. 


I'n;.  186.— The  left  auricle  and  ventricle  opened  and 
pari  of  their  walls  removed  to  show  their  cavities 
(Allen  Thomson).  1,  right  pulmonary  vein  cut 
short;  V,  cavity  of  left  auricle;  3,  thick  wall  of 
left  ventricle  ;  4,  portion  of  the  same  with  pap- 
illary muscle  attached  ;  5,  5',  the  other  papillary 
muscles;  0,  one  segment  of  the  mitral  valve;  7, 
in  aorta  is  placed  over  the  semilunar  valves. 


THE  CIRCULATION   OF   TI1E   BLOOD. 


217 


It  will  be  found  very  helpful  to  perform  some  of  the  dissec- 
tions under  water,  and  by  the  use  of  this  or  some  other  fluid 
the  action  of  the  valves  may  be  learned  as  it  can  in  no  other 
way.  By  a  little  manipulation  the  heart  may  be  so  held  that 
water  may  be  poured  into  the  orifices,  prepared  by  a  removal 
of  a  portion  of  the  blood-vessels  or  the  auricles,  when  the  valves 
may  be  seen  closing  together,  and  thus  revealing  their  action  in 
a  way  which  no  verbal  or  pictorial  representation  can  do  at  all 
adequately. 

A  heart  thoroughly  boiled  and  allowed  to  get  cold  shows,  on 
being  pulled  somewhat  apart,  the  course,  attachment,  and  other 


■It     T^m.v.1 


JRAV 


Fio.  187. -View  of  the  orifices  of  the  heart  from  below,  the  whole  of  the  ventricles 
having  been  cut  away  (after  Huxley).  JRAV,  right  auriculo-ventricular  orifice, 
surrounded  bv  the  three  flaps,  t.  v.  1,  t.  v.  2.  t.  v.  3,  of  the  tricuspid  valve,  which  are 
stretched  by  weights  attached  to  the  chorda  tendinece.  LAV.  left  auriculo-ven- 
tricular orifice,  etc.  PA.  orifice  of  the  pulmonary  artery,  the  semilunar  valves 
represented  as  having  met  and  closed  together.    A  0,  orifice  of  the  aorta. 


features  of  the  fibers  very  well,  as  also  the  skeleton  of  the  organ, 
which  may  be  readily  separated. 

When  this  has  all  been  done,  the  half  is  not  yet  accom- 
plished. A  visit  to  an  abattoir  will  now  repay  amply  for  the 
time  spent.  Animals  are  there  killed  and  eviscerated  so  rapidly 
that  an  observer  may  not  only  gain  a  good  practical  acquaint- 
ance with  the  relations  of  the  heart  to  other  parts,  but  may 
often  see  the  organ  still  living  and  exemplifying  that  action 


218 


COMPARATIVE   PHYSIOLOGY. 


peculiar  to  it  as  it  gradually  approaches  quiescence  and  death 
— a  matter  of  the  utmost  importance. 

If  the  student  will  then  compare  what  he  has  learned  of  the 
mammalian  heart  in  this  way  with  the  behavior  of  the  heart 
of  a  frog-,  snake,  fish,  turtle,  or  other  animal  that  may  be  killed 
after  brief  ether  narcosis,  without  cessation  of  the  heart's  ac- 
tion, he  will  have  a  broader  basis  for  his  cardiac  physiology 
than  is  usual ;  and  we  think  we  may  promise  the  medical  stu- 
dent, who  will  in  this  and  other  ways  that  may  occur  to  him 
supplement  the  usual  work  on  the  human  cadaver,  a  pleasure 
and  profit  in  the  study  of  heart-disease  which  come  in  no 
other  way. 

With  the  view  of  assisting  the  observation  of  the  student 
as  regards  the  heart  of  the  mammal,  we  would  call  special  atten 
tion  to  the  following  points  among  others :  Its  method  of  sus- 
pension, chiefly  by  its  great  vessels  ;  the  strong  fibrous  frame- 
work for  the  attachment  of  valves,  vessels,  and  muscle-fibers; 
the  great  complexity  of  the  arrangement  of  the  latter;  the 
various  lengths,  mode  of  attachment,  and  the  strength  of  the 

PA 


Fig.  188.— Orifices  of  the  heart  seen  from  above,  after  the  auricles  and  great  vessels 
had  been  cut  awav  (after  Huxley).  PA .  pulmonary  artery  with  its  semilunar  valves. 
Ao,  aorta  in  a  similar  condition.  RAV,  right  auriculo-ventricular  orifice,  with 
m.  v.  1  and  2  flaps  of  mitral  valve:  b.  style  passed  into  coronary  vein.  On  the  left 
part  of  LA  Fthe  section  of  the  auricle  is  carried  through  the  auricular  appendage, 
hence  the  toothed  appearance  due  to  the  portions  in  relief  cut.across. 

inelastic  chordae  tendinese;  the  papillary  muscles,  which  doubt- 
less act  at  the  moment  the  valves  flap  back,  thus  preventing 


THE  CIRCULATION  OF  THE  BLOOD.  219 

the  latter  being  carried  too  far  toward  the  auricles,  the  pocket- 
ing action  of  the  semilunar  valves  with  their  strong  margin 
and  meeting  nodules  {corpora  Arantii) ;  the  relative  thickness 
of  auricles  and  ventricles,  and  the  much  greater  thickness  of 
the  walls  of  the  left  than  of  the  right  ventricle— differences 
which  are  related  to  the  work  these  parts  perform. 

The  latter  may  be  well  seen  by  making  transverse  sections 
of  the  heart  of  an  animal,  especially  one  that  has  been  bled  to 
death,  which  specimen  also  shows  how  the  contraction  of  the 
heart  obliterates  the  ventricular  cavity. 

It  will  also  be  well  worth  while  to  follow  up  the  course 
of  the  coronary  arteries,  noting  especially  their  point  of 
origin. 

The  examination  of  the  valves  of  the  smaller  hearts  of  cold- 
blooded animals  is  a  matter  of  greater  difficulty  and  is  facili- 
tated by  dissection  under  water  with  the  help  of  a  lens  or  dis- 
secting microscope ;  but  even  without  these  instruments  much 
may  be  learned,  and  certainly  that  the  valves  are  relatively  to 
those  of  the  mammalian  heart  imperfectly  developed,  will  be- 
come very  clear. 

CIRCULATION   OF   THE  BLOOD   IN  THE    MAMMAL. 

It  is  highly  important  and  quite  possible  in  studying  the 
circulation  to  form  a  series  of  mental  pictures  of  what  is  trans- 
piring. It  will  be  borne  in  mind  that  there  is  a  set  of  elastic 
tubes  of  relatively  thick  walls,  standing  open  when  cut  across, 
dividing  into  smaller  and  smaller  branches,  and  finally  ending 
in  vessels  of  more  than  cobweb  fineness,  and  opening  out  into 
others,  that  become  larger  and  larger  and  fewer  and  fewer,  till 
they  are  gathered  up  into  two  of  great  size  which  form  the  right 
auricle.  The  larger  pipes  consist  everywhei'e  of  elastic  tissue 
proper,  muscular  tissue  (itself  elastic),  fibrous  tissue,  and  a  flat 
epithelial  lining,  so  smooth  that  the  friction  therefrom  must  be 
minimal  as  the  blood  flows  over  it. 

The  return  tubes  or  veins  are  like  the  arteries,  but  so  thin 
that  their  walls  fall  together  when  cut  across.  They  are  differ- 
ent from  all  the  other  blood-tubes  in  that  they  possess  valves 
opening  toward  the  heart  throughout  their  course.  The  veins 
are  at  least  twice  as  numerous  as  the  arteries,  and  their  capacity 
many  times  greater.  The  small  vessels  or  capillaries  are  so 
abundant  and  wide-spread  that,  as  is  well  known,  the  smallest 


220 


COMPARATIVE  PHYSIOLOGY. 


cut  anywhere  gives  rise  to  a 
flow  of  blood,  owing  to  sec- 
tion of  some  of  these  tubes, 
which,  it  will  be  remembered, 
are  not  visible  to  the  unaided 
eye.  It  is  estimated  that  their 
united  area  is  several  hun- 
dred (500  to  800)  times  that  of 
the  arteries. 

If  we  suppose  the  epithe- 
lial lining  pushed  out  of  a 
small  artery  we  have,  so  far 
as  structure  alone  goes,  a 
good  idea  of  a  capillary — i.  e., 
its  walls  are  but  one  cell 
thick,  and  these  cells  though 
loug  are  extremely  thin,  so 
that  it  is  quite  easy  to  under- 
stand how  it  is  that  the  amoe- 
boid corpuscles  can,  under 
certain  circumstances,  push 
tbeir  way  through  its  proba- 
bly semi-fluid  walls. 

From  what  has  been  said, 
it  will  be  seen  that  the  whole 
collection  of  vascular  tubes 
may  be  compared  to  two  inverted  funnels  or  cones  with  the 


Fig.  189.— Various  layers  of  the  walls  of  a 
small  artery  (Landois).  e,  endothelium; 
i.  e,  internal  elastic  lamina;  c.  m,  circu- 
lar muscular  fibers  of  the  middle  coat; 
c.  t,  connective  tissue  of  the  outer  coat, 
or  T.  adventitia. 


Fig.  190. 


Fio.  191. 


Fig.  190.— Vein  with  valves  lying  open  (Dalton). 

In.    191.— Vein  with  valves  closed,  the  blood  passing  on  by  a  lateral  branch  below 
(Dalton). 


THE   CIRCULATION   OF   THE   BLOOD. 


221 


wIfi  iqo  _CaDillarv  blood-vessels  (Landois).  The  cement  substance  between  the  en- 
dothelium h'^Len  rendered  dark  by  silver  nitrate,  and  the  nuclei  made  prominent 
by  staining. 

smaller  end  toward  the  heart  aud  the  widest  portions  repre- 
senting the  capillaries. 


Pig.  193—Diagram  to  illustrate  the  relative  proportions  of  the  ^egate  sectional  we.; 

of  the  different  parts  of  the  vascular  system  (after  \eo).    A,  aorta,  C,  capillars . 
V,  veins. 


222 


COMPARATIVE   PHYSIOLOGY, 


THE   ACTION   OF   THE   MAMMALIAN   HEART. 

What  takes  place  may  be  thus  very  briefly  stated  :  The 
right  auricle  contracting-  squeezes  the  blood  through  the  au- 
ricular-ventricular opening  into  the  right  ventricle,  never  quite 


Superior  Vena 
Cava. 


Inferior  Vena 
Cava. 


Capillaries  of  the 
Head,  etc. 


Pulmonary  Ca- 
pillaries. 


Capillaries  of 
Trunk  and 
Lower  Ex- 
tremities. 


Fk;.  194.— Diagram  of  the  circulation.  The  arrows  indicate  the  course  of  the  blood. 
Though  the  pulmonary,  the  lower  and  the  upper  parts  of  the  systemic  circulation 
are  represented  so  as  to  show  the  distinctness  of  each,  it  will  be  also  apparent  that 
they  are  not  independent.  Relative  size  of  different  parts  of  the  system  is  only 
very  generally  indicated. 

emptying  itself  probably;  immediately  after  the  right  ventricle 
contracts,  by  which  its  valves  are  brought  into  sudden  tension 
and  opposition,  thus  preventing  reflux  into  the  auricle  ;  while 
the  blood  within  it  takes  the  path  of  least  resistance,  and  the 


THE  CIRCULATION  OP  THE  BLOOD.  223 

only  one  open  to  it  into  the  pulmonary  artery,  and  by  its 
branches  is  conveyed  to  the  capillaries  of  the  lungs,  from  which 
it  is  returned  freed  from  much  of  its  carbonic  anhydride  and 
replenished  with  oxygen,  to  the  left  auricle,  whence  it  proceeds 
in  a  similar  manner  into  the  great  arterial  main,  the  aorta,  for 
general  distribution  throughout  the  smaller  arteries  and  the 
capillaries  to  the  most  remote  as  well  as  the  nearest  parts,  from 
which  it  is  gathered  up  and  returned  laden  with  many  impuri- 
ties, and  robbed  of  a  large  proportion  of  its  useful  matters,  to 
the  right  side  of  the  heart. 

It  will  be  remembered  that  corresponding  subdivisions  of 
each  side  of  the  heart  act  simultaneously,  and  that  any  decided 
departure  from  this  harmony  of  rhythm  would  lead  to  serious 
disturbance. 


THE  VELOCITY  OF  THE  BLOOD  AND  BLOOD-PRESSURE. 

If  the  relative  capacity  and  arrangement  of  the  various  parts 
of  the  circulatory  system  be  as  has  been  represented,  it  follows 
that  we  may  predict  with  some  confidence,  apart  from  experi- 
ment, what  the  speed  of  the  flow  and  the  vascular  tension  must 
be  in  different  parts  of  the  course  of  the  circulation. 

We  should  suppose  that,  in  the  nature  of  the  case,  the  veloc- 
ity would  be  greatest  in  the  large  arteries,  gradually  diminish 
to  the  capillaries,  in  which  it  would  be  much  the  slowest  and, 
getting  by  degrees  faster,  would  reach  a  speed  in  the  largest 
veins  approaching  that  of  the  corresponding  arteries. 

The  methods  of  determining  the  velocity  of  the  blood-stream 
have  not  entirely  surmounted  the  difficulties,  but  they  do  give 
results  in  harmony  with  the  above-noted  anticipations. 

The  area  of  the  great  aortic  trunk  being  so  much  less  than 
that  of  the  capillaries,  the  flow  in  that  vessel  we  should  expect 
to  be  very  much  swifter  than  in  the  arterioles  or  the  capillaries. 
Moreover  there  must  be  a  great  difference  in  the  velocity  during 
cardiac  systole  and  diastole,  and  according  as  the  beat  of  the 
heart  is  forcible  or  otherwise.  But  apart  from  these  more  ob- 
vious differences,  there  are  variations  depending  on  complex 
changes  in  the  peripheral  circulation,  owing  to  the  frequent 
variations  in  the  diameter  of  the  arterioles  in  different  parts, 
as  well  as  differences  in  the  resistance  offered  by  the  capillaries, 
the  causes  of  which  are  but  ill  understood,  though  less  obscure, 
we  think,  than  they  are  often  represented  to  be.     Since  for  the 


224  COMPARATIVE  PHYSIOLOGY. 

maintenance  of  the  circulation,  the  quantity  of  blood  entering 
and  leaving  the  heart  must  be  equal,  in  consequence  of  the  sec- 
tional area  of  the  great  veins  that  enter  the  heart  being  greater 
than  that  of  the  aorta,  it  follows  that  the  venous  flow  even  at  its 
quickest  is  necessarily  slower  than  the  arterial. 

Comparative. — There  must  be  great  variations  in  velocity  in 
different  animals,  as  such  measurements  as  have  been  made 
demonstrate.  Thus,  in  the  carotid  of  the  horse,  the  speed  of 
the  blood-current  is  calculated  as  about  306  mm.,  in  the  dog  at 
from  205  to  357  mm.  These  results  can  not  be  considered  as 
more  than  fair  approximations. 

Highly  important  is  it  to  note  that  the  rate  of  flow  in  the 
capillaries  of  all  animals  is  very  slow  indeed,  not  being  as  much 
as  1  mm.  in  a  second  in  the  larger  mammals.  The  time  occupied 
by  the  circulation  is  also,  of  course,  variable,  being  as  a  rule 
shorter  the  smaller  the  animals.  As  the  result  of  a  number  of 
calculations,  though  by  methods  that  are  more  or  less  faulty, 
the  following  law  may  be  laid  down  as  meeting  approximately 
the  facts  so  far  as  warm-blooded  animals  are  concerned. 

The  circulation  is  effected  by  27  beart-beats  ;  thus  for  a  man 
with  a  pulse  of  81,  the  time  occupied  in  the  completion  of  the 
course  of  the  blood  from  and  to  the  heart  would  be  f|  =  3 ;  i.  e., 
the  circulation  is  completed  three  times  in  one  minute,  or  its 
period  is  twenty  seconds ;  and  it  is  to  be  well  borne  in  mind 
that  by  far  the  greater  part  of  this  time  is  occupied  in  traversing 
the  capillaries. 

THE    CIRCULATION   UNDER  THE   MICROSCOPE. 

There  are  few  pictures  more  instructive  and  impressive  than 
a  view  of  the  circulation  of  the  blood  under  the  microscope. 
It  is  well  to  have  similar  preparations,  one  under  a  low  power 
and  another  under  a  magnification  of  300  to  500  diameters. 
With  the  former  a  view  of  arterioles,  veins,  and  capillaries  may 
be  obtained  at  once.  Many  different  parts  of  animals  may  be 
used,  as  the  web  of  the  frog's  foot,  its  tongue,  lung,  or  mesen- 
tery ;  the  gill  or  tail  of  a  small  fish,  tadpole,  etc. 

The  relative  size  of  the  vessels  ;  the  speed  of  the  blood  flow; 
the  greater  velocity  of  the  central  part  of  the  stream ;  the  aggre- 
gation of  colorless  corpuscles  at  the  sides  of  the  vessels,  and  the 
occasional  passage  of  one  through  a  capillary  wall,  when  the 
exposure  has  lasted  some  time;  the  crowding  of  the  red  cells; 


THE   CIRCULATION  OF  THE   BLOOD. 


225 


their  plasticity ;  the  small  size  of  some  of  the  capillaries,  barely 
allowing  the  corpuscles  to  be  squeezed  through;  the  changes  in 
the  velocity  of  the  current,  especially  in  the  capillaries ;  its  pos- 
sible arrest  or  retrocession ;  the  velocity  in  one  so  much  greater 
than  in  its  neighbor,  without  very  obvious  cause — all  this  and 


Fig.  195. — Portion  of  the  web  of  a  frog's  foot  as  seen  under  a  low  magnifying  power, 
showing  the  blood-vessels,  and  in  one  corner  the  pigment-spots  (after  Huxley),  a, 
small  arteries  (arterioles);  v,  small  veins.  The  smaller  vessels  are  the  capillaries. 
The  course  of  the  blood  is  indicated  by  arrows. 

much  more  forms,  as  we  have  said,  a  remarkable  lesson  for  the 
thinking  student.  This,  like  all  microscopic  views,  especially 
if  motion  is  represented,  has  its  fallacies.  It  is  to  be  remem- 
bered that  the  movements  are  all  magnified,  or  else  one  is  apt 
to  suppose  the  capillary  circulation,  extremely  rapid,  whereas 
it  is  like  that  of  the  most  sluggish  part  of  a  stream,  and  very 
irregular. 

15 


226 


COMPARATIVE  PHYSIOLOGY. 


Fig.  196.— Circulation  in  the  web  of  a  frog's  foot  (Wagner).  V,  venous  trunk  com- 
posed of  the  three  principal  branches  (v.  v,  v)  covered  with  a  plexus  of  smaller  ves- 
sels.   The  whole  is  dotted  over  with  pigment  masses. 


THE   CHARACTERS   OF  THE   BLOOD-FLOW. 

If  an  artery  be  opened,  the  blood  is  seen  to  flow  from  it  in 
a  constant  stream,  with  periodic  exaggerations,  which,  it  is 
found,  answer  to  the  heart-beats  ;  in  the  case  of  veins  and 
capillaries  the  flow  is  also  constant,  but  shows  none  of  the 
spurting  of  the  arterial  stream,  nor  has  the  cardiac  beat  appar- 
ently an  equal  modifying  effect  upon  it. 

We  have  already  explained  why  the  flow  should  be  constant, 
though  it  would  be  well  to  be  clearer  as  to  the  peripheral  re- 
sistance. The  amount  of  friction  from  linings  so  smooth  as 
those  of  the  blood-vessels  can  not  be  considerable.  Whence, 
then,  arises  that  friction  which  keeps  the  arterial  vessels  always 
distended  by  its  backward  influence  ?  The  microscopic  study 
of  the  circulation  helps  to  answer  this  question.  The  plas- 
ticity of  the  corpuscles  and  of  the  vessel  walls  themselves  must 
be  taken  into  account,  in  consequence  of  which  a  dragging 
influence  is  exerted  whenever  the  corpuscles  touch  the  wall, 
which  must  constantly  happen  with  vast  numbers  of  them  in 
the  smallest  vessels  and  especially  in  the  capillaries.  The 
arrangement  of  capillaries  into  a  mesh-work,  must  also,   in 


THE  CIRCULATION  OF  THE  BLOOD.  227 

consequence  of  so  many  angles,  be  a  source  of  much  fric- 
tion. 

The  action  of  the  corpuscles  on  one  another  may  be  com- 
pared to  a  crowd  of  people  hurrying  along  a  narrow  passage — 
the  obstruction  comes  from  interaction  of  a  variety  of  forces, 
owing  to  the  crowd  itself  rather  than  the  nature  of  the  thor- 
oughfare. We  must  set  down  a  great  deal  to  the  influence  of 
the  corpuscles  on  one  another,  as  they  are  carried  along  accord- 
ing to  mechanical  principles  ;  but,  as  we  shall  see  later,  other 
and  more  subtle  factors  play  a  part  in  the  capillary  circulation. 
Owing  to  the  peripheral  resistance  and  the  pumping  force  of 
the  heart,  the  arteries  become  distended,  so  that,  during  cardiac 
diastole,  their  recoil,  owing  to  the  closure  of  the  semilunar 
valves,  forces  on  the  blood  in  a  steady  stream.  It  follows,  then, 
that  the  main  force  of  the  heart  is  spent  in  distending  the 
arteries,  and  that  the  immediate  propelling  force  of  the  circu- 
lation is  the  elasticity  of  the  arteries  hi  which  the  heart  stores 
up  the  energy  of  its  systole  for  the  moment. 

BLOOD-PRESSURE. 

Keeping  in  mind  our  schematic  representation  of  the  circu- 
lation, we  should  expect  that  the  blood  must  exercise  a  certain 
pressure  everywhere  throughout  the  vascular  system ;  that  this 
blood-pressure  would  be  highest  in  the  heart  itself ;  considera- 
ble in  the  whole  arterial  system,  though  gradually  diminishing 
toward  the  capillaries,  in  which  it  would  be  feeble ;  lower  still 
in  the  smaller  veins ;  and  at  its  minimum  where  the  great  veins 
enter  the  heart.  Actual  experiments  confirm  the  truth  of  these 
views ;  and,  as  the  subject  is  one  of  considerable  importance,  we 
shall  direct  attention  to  the  methods  of  estimating  and  record- 
ing an  animal's  blood-pressure. 

First  of  all,  the  well-known  fact  that,  when  an  artery  is  cut, 
the  issuing  stream  spurts  a  certain  distance,  as  when  a  water- 
main,  fed  from  an  elevated  reservoir,  bursts,  or  a  hjTdrant  is 
opened,  is  itself  a  proof  of  the  existence  of  blood-pressure,  and 
is  a  crude  measure  of  the  amount  of  the  pressure. 

One  of  the  simplest  and  most  impressive  ways  of  demon- 
strating blood-pressure  is  to  connect  the  carotid,  femoral,  or 
other  large  artery  of  an  animal  by  means  of  a  small  glass  tube 
(drawn  out  in  a  peculiar  manner  to  favor  insertion  and  reten- 
tion by  ligature  in  the  vessel),  known  as  a  cannula,  by  rubber 


228 


COMPARATIVE  PHYSIOLOGY. 


Fig.  197. 


THE  CIRCULATION   OB"   THE   BLOOD.  229 

Fi«.  19?.— Apparatus  used  in  making  a  blood-pressure  experiment  (after  Foster)  //.  b. 
pressure-bottle,  elevated  so  as  to  raise  the  pressure  several  inches  of  mercury,  as 
seen  in  the  manometer  (m)  below.  It  contains  a  saturated  solution  of  sodium  car- 
bonate; r.  t,  rubber  tube  connecting  the  pb  with  the  leaden  tube:  /.  t,  tube  made 
of  lead,  so  as  to  be  pliable,  yet  have  rigid  walls;  t«.  c,  a  stop-cock,  the  top  of  which 
is  removable,  to  allow  escape  of  bubbles  of  air;  p,  the  pen,  writing  on  the  roll  of 
paper,  r.  The  former  floats  on  the  mercury;  m,  the  manometer,  the  shaded  por- 
tion of  the  bent  tube  denoting  the  mercury,  the  rest  is  tilled  with  a  fluid  unfavor- 
able to  the  coagulation  of  the  Dlood,  and  derived  from  the  pressure-bottle;  ca,  the 
carotid,  in  which  is  placed  the  cannula,  and  below  the  latter  a  forceps,  which  may 
be  removed  when  the  blood-pressure  is  to  be  actually  measured.  The  registration 
of  the  height,  variation,  etc.,  of  blood-pressure,  is  best  made  on  a  continuous  roll 
of  paper,  as  seen  in  Fig.  198. 

tubing,  with  a  long  glass  rod  of  bore  approaching  that  of  the 
artery  opened,  into  which  the  blood  is  allowed  to  flow  through 
the  above-mentioned  connections,  while  it  is  maintained  in  a 
vertical  position. 

To  prevent  the  rapid  coagulation  of  the  blood  in  such  ex- 
periments, it  is  customary  to  fill  the  cannula  and  other  tubes 
to  a  certain  extent,  at  least,  with  a  solution  of  some  salt  that 
tends  to  retard  coagulation,  such  as  sodium  carbonate  or  bicar- 
bonate, magnesium  sulphate,  etc.  If  other  connections  are 
made  in  a  similar  way  with  smaller  arteries  and  veins,  it  may 
be  seen  that  the  height  of  the  respective  columns  representing 
the  blood-pressure,  varies  in  each  and  in  accordance  with  ex- 
pectations. 

While  all  the  essential  facts  of  blood-pressure  and  many 
others  may  be  illustrated  by  the  above  simple  methods,  it  is  inad- 
equate when  exact  measurements  are  to  be  made  or  the  results 
to  be  recorded  for  permanent  preservation ;  hence  apparatus  of 
a  somewhat  elaborate  kind  has  been  devised  to  accomplish  these 
.  purposes. 

The  graphic  methods  are  substantially  those  already  ex- 
plained in  connection  with  the  physiology  of  muscle ;  but,  since 
it  is  often  desirable  to  maintain  blood-pressure  experiments 
for  a  considerable  time,  instead  of  a  single  cylinder,  a  series  so 
connected  as  to  provide  a  practically  endless  roll  of  paper  (Fig. 
198)  is  employed. 

When,  in  the  sort  of  experiments  referred  to  above,  the 
height  of  the  fluid  used  in  the  glass  tube  to  prevent  coagula- 
tion just  suffices  to  prevent  outflow  from  the  artery  into  the 
connections,  we  have,  of  course,  in  this  a  measure  of  the  blood- 
pressure;  however,  it  is  convenient  in  most  instances  to  use 
mercury,  contained  in  a  glass  tube  bent  in  the  form  of  a  U.  for 
a  measure,  as  shown  in  the  subjoined  illustration.  It  is  also 
desirable,  in  order  to  prevent  outflow  of  the  blood  into  the 
apparatus,  to  get  up  a  pressure  in  the  U-tube  or  manometer  as 


230 


COMPARATIVE   PHYSIOLOGY. 


near  as  may  be  equal  to  that  of  the  animal  to  be  employed  in 
the  experiment.  This  may  be  effected  in  a  variety  of  ways,  one 
of  the  most  convenient  of  which  is  by  means  of  a  vessel  con- 
taining some  saturated  sodium  carbonate  or  similar  solution  in 
connection  with  the  manometer. 

It  is  important  that  the  pressure  should  express  itself  as 
directly  and  truthfully  on  the  mercury  of  the  manometer  as 
possible,  hence  the  employment  of  a  tube  with  rigid  walls,  yet 
capable  of  being  bent  readily  in  different  directions  for  the  sake 
of  convenience. 

Mercury,  on  account  of  its  inertia,  is  not  free  from  objec- 
tion ;  and  when  very  delicate  variations  in  the  blood-pressure — 


Pia.  198.— Large  kymograph,  with  continuous  roll  of  paper  (Foster).  The  clock-work 
machinery  unrolls  the  paper  from  the  roll  C,  carries  it  smoothly  over  the  cylinder 
B,  and  then  winds  it  up  into  the  roll  A.  Two  electro-magnetic  markers  are  seen 
in  position  recording  intervals  of  time  on  the  moving  roll  of  paper.  A  manometer 
may  !><•  fixed  in  any  convenient  position. 

e.  g.,  feeble  pulse-beats — are  to  be  indicated,  it  fails  to  express 
them,  in  which  case  other  fluids  may  be  employed. 

It  will  be  noted  that  when  an  ordinary  cannula  is  used, 
inserted  as  it  is  lengthwise  into  the  blood-vessel,  the  pressure 
recorded  is  not  that  on  the  side  of  the  vessel  into  which  it  is 
inserted  as  when  a  —\  -  piece  is  used,  but  of  the  vessel,  of  which 
the  one  in  question  is  a  branch.  The  blood-pressure,  in  the 
main  arterial  trunk,  for  example,  must  depend  largely  on  the 


THE  CIRCULATION  OF  THE   BLOOD.  231 

force  of  the  heart-beat ;  consequently  it  would  be  expected,  and 
it  is  actually  found,  that  the  pressure  varies  for  different  ani- 
mals, size  having,  of  course,  in  most  instances  a  relation  to 
the  result.  It  has  been  estimated  that  in  the  carotid  of  the  horse 
the  arterial  pressure  is  150  to  200  mm.  of  mercury,  of  the  dog 
100  to  175,  of  the  rabbit  50  to  90.  Man's  blood-pressure  is  not 
known,  but  is  probably  high,  we  may  suppose  not  less  than 
150  to  200  mm. 

After  the  fact  that  there  is  a  certain  considerable  blood- 
pressure,  the  other  most  important  one  to  notice  is  that  this 
blood-pressure  is  constantly  varying  during  the  experiment, 
and,  as  we  shall  give  reason  to  believe,  in  the  normal  animal ; 
and  to  these  variations  and  their  causes  we  shall  presently  turn 
our  attention. 


THE  HEART. 

The  heart,  being  one  of  the  great  centers  of  life,  to  speak 
figuratively,  it  demands  an  unusually  close  study. 

THE   CARDIAC   MOVEMENTS. 

There  is  no  special  difficulty  in  ascertaining  the  outlines  of 
the  heart  by  means  of  percussion  on  either  the  dead  or  the 
living  subject.  Quite  otherwise  is  it  with  the  changes  in  form 
which  accompany  cardiac  action.  Attempts  have  been  made 
to  ascertain  the  alterations  in  position  of  the  heart  with  respect 
to  other  parts,  and  especially  its  own  alterations  in  shape  dur- 
ing a  systole,  the  chest  being  unopened,  by  the  use  of  needles 
thrust  into  its  substance  through  the  thoracic  walls ;  but  the  re- 
sults have  proved  fallacious.  Again,  casts  have  been  made  of 
the  heart  after  death,  in  a  condition  of  moderate  extension,  prior 
to  rigor  mortis ;  and  also  when  contracted  by  a  hardening  fluid. 
These  methods,  like  all  others  as  yet  employed,  are  open  to  seri- 
ous objections. 

Following  the  rapidly  beating  heart  of  the  mammal  with 
the  eye  produces  uncertainty  and  confusion  of  mind. 

It  may  be  very  confidently  said  that  the  mode  of  contrac- 
tion of  the  hearts  of  different  groups  of  vertebrates  is  variable, 
though  it  seems  highly  probable  that  the  divergences  in  mam- 
mals are  slight.  The  most  that  can  be  certainly  affirmed  of  the 
mammalian  heart  is,  that  during  contraction  of  the  ventricles 


232  COMPARATIVE   PHYSIOLOGY. 

they  become  more  conical ;  that  the  long  diameter  is  not  appre- 
ciably altered ;  that  the  antero-posterior  diameter  is  lengthened ; 
and  that  the  left  ventricle  at  least  turns  on  its  own  axis  from 
left  to  right.  This  latter  may  be  distinctly  made  out  by  the  eye 
in  watching  the  heart  in  the  opened  chest. 

THE  IMPULSE  OF  THE  HEART. 

When  one  places  his  hand  over  the  region  of  the  heart  in 
man  and  other  mammals,  he  experiences  a  sense  of  pressure 
varying  with  the  part  touched,  and  from  moment  to  moment. 
Instruments  constructed  to  convey  this  movement  to  recording 


Fig,,  199.— Marey's  cardiac  sound,  which  may  be  used  to  explore  the  chambers  of  the 
heart  (after  Foster),  a  is  made  of  rubber  stretched  over  a  wire  framework,  with 
metallic  supports  above  and  below;  b  is  a  long  tube. 

levers  also  teach  that  certain  movements  of  the  chest  wall  cor- 
respond with  the  propagation  of  the  pulse,  and  therefore  to  the 
systole  of  the  heart.  It  can  be  recognized,  whether  the  hand 
or  an  instrument  be  used,  that  all  parts  of  the  chest  wall  over 
the  heart  are  not  equally  raised  at  the  one  instant.  If  the  beat- 
ing heart  be  held  in  the  hand,  it  will  be  noticed  that  during 
systole  there  is  a  sudden  hardening.  The  relation  of  the  apex 
to  the  chest  wall  is  variable  for  different  mammals,  and  with 
different  positions  of  the  body  in  man. 

As  a  result  of  the  investigation  which  this  subject  has  re- 
ceived, it  may  be  inferred  that  the  sudden  tension  of  the  heart, 
owing  to  the  ventricle  contracting  over  its  fluid  contents, 
causes  in  those  cases  in  which  during  diastole  the  ventricle 
lies  against  the  chest  wall,  a  sense  of  pressure  beneath  the 
hand,  which  is  usually  accompanied  by  a  visible  movement 
upward  in  some  part  of  the  thoracic  wall,  and  downward  in 
adjacent  parts. 

It  will  not  be  forgotten  that  the  heart  lies  in  a  pericardial 
sac,  moistened  with  a  small  quantity  of  albuminous  fluid  ;  and 
that  by  this  sac  the  organ  is  tethered  to  the  walls  of  the  chest 
by  its  mediastinal  fastenings  ;  so  that  in  receding  from  the 
chest  wall  the  latter  may  be  drawn  after  it  ;  though  this  might 


THE   CIRCULATION  OF  THE   BLOOD. 


233 


Cardiac 
impulse. 


Fig.  200.— Simultaneous  tracings  from  the  interior  of  the  right  auricle,  from  the  inte- 
rior of  the  right  ventricle,  and  of  the  cardiac  impulse  in  the  horse  (after  Chauveau 
and  Marey).  Tracings  to  be  read  from  left  to  right,  and  the  references  above  are 
in  the  order  from  top  to  bottom.  A  complete  cardiac  cycle  is  included  between 
the  thick  vertical  lines  I  and  II.  The  thin  vertical  lines  indicate  tenths  of  a  sec- 
ond. The  gradual  rise  of  pressure  within  the  ventricle  (middle  tracing)  during 
diastole,  the  sudden  rise  with  the  systole,  its  maintenance  with  oscillations  for  an 
appreciable  time,  its  sudden  fall,  etc.,  are  all  well  shown.  There  is  disagreement 
as  to  the  exact  meaning  of  the  minor  curves  in  the  larger  ones. 

also  follow  from  the  intercostal  muscles  being  simply  unsup- 
ported when  the  heart  recedes. 


INVESTIGATION    OP    THE    HEART-BEAT    FROM 
WITHIN. 

By  the  use  of  apparatus,  introduced  within  the  heart  of  the 
mammal  and  reporting  those  changes  susceptible  of  graphic 
record,  certain  tracings  have  been  obtained  about  the  details  of 
which  there  are  uncertainty  and  disagreement,  though  they 
seem  to  establish  the  nature  of  the  main  features  of  the  cardiac 
beat  clearly  enough.  An  interpretation  of  such  tracings  in  tbe 
light  of  our  general  and  special  knowledge  warrants  the  follow- 
ing statement. 

1.  Both  auricular  and  ventricular  systole  are  sudden,  but  the 
latter  is  of  very  much  greater  duration. 

2.  While  the  chest  wall  feels  the  ventricular  systole,  the 
auriculo-ventricular  valves  shield  the  auricle  from  its  shock. 

3.  During  diastole  in  both  chambers  tbe  pressure  rises  gradu- 


234  COMPARATIVE   PHYSIOLOGY. 

ally  from,  the  inflow  of  "blood ;  and  the  auricular  contraction 
produces  a  brief,  decided,  though  but  slight  rise  of  pressure  in 
the  ventricles. 

4.  The  onset  of  the  ventricular  systole  is  rapid,  its  maximum 
pressure  suddenly  reached,  and  its  duration  considerable. 

The  relations  of  these  various  events,  their  duration  and 
the  corresponding  movements  of  the  chest  wall,  may  be  learned 
by  a  study  of  the  above  tracing  which  the  student  will  find 
worthy  of  his  close  attention. 

THE   CARDIAC   SOUNDS. 

Two  sounds,  differing  in  pitch,  duration,  and  intensity,  may 
be  heard  over  the  heart  when  the  chest  is  opened  and  the  heart 
listened  to  by  means  of  a  stethoscope.  These  sounds  may  also 
be  heard,  and  present  the  same  characters  when  the  heart  is 
auscultated  through  the  chest  wall ;  hence  the  cardiac  impulse 
can  take  no  essential  part  in  their  production. 

The  sounds  are  thought  to  be  fairly  well  represented,  so  far 
as  the  human  heart  is  concerned,  by  the  syllables  lub,  dup  ;  the 
first  sound  being  longer,  louder,  lower-pitched,  and  '"  booming  " 
in  quality ;  the  second  short,  sharp,  and  high-pitched. 

In  the  exposed  heart,  the  first  sound  is  heard  most  distinctly 
over  the  base  of  the  organ  or  a  little  below  it ;  while  the  sec- 
ond is  communicated  most  distinctly  over  the  roots  of  the  great 
vessels — that  is  to  say,  both  sounds  are  heard  best  over  the 
auriculo-ventricular  and  semilunar  valves  respectively.  When 
the  chest  Wall  intervenes  between  the  heart  and  the  ear,  it  is 
found  that  the  second  sound  is  usually  heard  most  distinctly 
over  the  second  costal  cartilage  on  the  right ;  and  the  first  in 
the  fifth  costal  interspace  where  the  heart's  impulse  is  also  often 
most  distinct.  In  these  situations  the  arch  of  the  aorta  in  the 
one  case,  and  the  ventricular  walls  in  the  other,  are  close  to  the 
situations  referred  to  during  the  cardiac  systole  ;  hence  it  is 
inferred  that,  though  the  sounds  do  not  originate  directly  be- 
neath these  spots,  they  are  best  propagated  to  the  chest  wall  at 
these  points.  Prior  to  the  study  of  the  heart  in  our  domestic 
animals  the  student  is  recommended  to  investigate  the  subject 
on  himself  by  means  of  a  double  stethoscope  or  on  another  per- 
son with  or  without  any  instruments. 

There  are,  however,  individual  differences,  owing  to  a  va- 
riety of  causes,  which  it  is  not  always  possible  to  explain  fully 


THE   CIRCULATION   OP  THE   BLOOD. 


235 


in  each  case,  but  owing  doubtless  in  great  part  to  variations  in 
anatomical  relations. 

The  Causes  of  the  Sounds  of  the  Heart.— There  is  general 
agreement  in  the  view  that  the  second  sound  is  owing  to  the 
closure  of  the  semilunar  valves  of  the  aortic  and  pulmonary 
vessels  ;  the  former,  owing  to  their  greater  tension  in  conse- 
quence of  the  higher  blood-pressure  in  the  aorta,  taking  much 
the  larger  share  in  the  production  of  the  sound,  as  may  be 
ascertained  by  listening  over  these  vessels  in  the  exposed  heart. 
When  these  valves  are  hooked  back,  the  second  sound  disap- 
pears, so  that  there  can  be  no  doubt  that  they  bear  some  impor- 
tant relation  to  the  causation  of  the  sound. 

In  regard  to  the  first  sound  of  the  heart  the  greatest  diversity 
of  opinion  has  prevailed  and  still  continues  to  exist.  The  fol- 
lowing among  other  views  have  been  advocated  by  physiolo- 
gists : 

1.  The  first  sound  is  caused  by  the  tension  and  vibration  of 
the  auriculo- ventricular  valves. 

2.  The  first  sound  is  owing  to  the  contractions  of  the  large 
mass  of  muscle  composing  the  ventricles. 

3.  The  sound  is  directly  traceable  to  eddies  in  the  blood. 


Fig.  201. 

Fig.  201.— Microscopic  appearance  of  fibers  from  the  heart.    The  cross-strite,  divisions 

(branching),  and  junctures  are  visible  (Landois). 
Fig.  202.— Muscular  fiber-cells  from  the  heart.    (1  x  425.)    a,  line  of  juncture  between 

two  cells;  b.  c,  branching  cells. 

But,  looking  at  the  whole  question  broadly,  is  it  not  unrea- 
sonable to  explain  the  sound  resulting  from  such  a  complex  act 


236  COMPARATIVE   PHYSIOLOGY. 

as  the  contraction  of  the  heart  and  what  it  implies  in  the  light 
of  any  single  factor  ?  That  such  narrow  and  exclusive  views 
should  have  been  propagated,  even  by  eminent  physiologists, 
should  admonish  the  student  to  receive  with  great  caution  ex- 
planations of  the  working  of  complex  organs,  based  on  a  single 
experiment,  observation,  or  argument  of  any  kind. 

The  view  we  recommend  the  student  to  adopt  in  the  light  of 
our  present  knowledge  is,  that  the  first  sound  is  the  result  of 
several  causative  factors,  prominent  among  which  are  the  sud- 
den tension  of  the  auriculo-ventricular  valves,  and  the  contrac- 
tion of  the  cardiac  muscle,  not  leaving  out  of  the  account  the 
possible  and  probable  influence  of  the  blood  itself  through 
eddies  or  otherwise;  nor  would  we  ridicule  the  idea  that  in 
some  cases,  at  all  events,  the  sound  may  be  modified  in  quality 
and  intensity  by  the  shock  given  to  the  chest  wall  during  sys- 
tole. 

ENDO-CARDIAC   PRESSURES. 

Bearing  in  mind  the  relative  extent  of  the  pulmonary  and 
systemic  portions  of  the  circulation,  we  should  suppose  that  the 
resistance  to  be  overcome  in  opening  the  aortic  valves  and  lift- 
ing the  column  of  blood  that  keeps  them  pressed  together, 
would  be  much  greater  in  the  left  ventricle  than  in  the  right ; 
or,  in  other  words,  that  the  intra-ventricular  pressure  of  the 
left  side  of  the  heart  would  greatly  exceed  that  of  the  right,  and 
this  is  confirmed  by  actual  experiment. 

That  there  should  be  a  negative  pressure  in,  say,  the  left 
ventricle,  follows  naturally  enough  from  the  fact  that  not  only 
are  the  contents  of  the  ventricle  expelled  with  great  sudden- 
ness, but  that  its  walls  remain  (see  Figs.  200  and  204)  pressed 
together  for  a  considerable  portion  of  the  time  occupied  by  the 
whole  systole ;  so  that  in  relaxation  it  follows  that  there  must 
be  an  empty  cavity  to  fill,  or  that  there  must  be  an  aspiratory 
effect  toward  the  ventricle;  hence  also  one  factor  in  the  closure 
of  the  semilunar  valves. 

It  thus  appears  that  the  heart  is  not  only  a  force-pump  but 
also  to  some  extent  a  suction-pump ;  and,  if  so,  the  aspirating 
effect  must  express  itself  on  the  great  veins,  lacking  valves  as 
they  do,  at  their  entrance  into  the  heart  ;  hence,  with  each 
diastole  the  blood  would  be  sucked  on  into  the  auricles,  a 
result  that  is  intensified  by  the  respiratory  movements  of  the 
thorax. 


THE  CIRCULATION   OF   THE   BLOOD. 


237 


Fig.  203. — Diagram  showing  the  relative  height  of  the  blood-pressure  in  different  parts 
of  the  vascular  system  (after  Yeo).  h,  heart;  a,  arterioles  ;  v,  small  veins;  A, 
arteries  ;  C,  capillaries  ;  V,  large  veins;  H,  V,  representing  the  zero-line,  i.  e.,  at- 
mospheric pressure  ;  the  blood-pressure  is  indicated  by  the  height  of  the  curve. 
The  numbers  on  the  left  give  the  pressure  approximately,  in  mm.  of  mercury. 

Relative  Time  occupied  by  the  Various  Phases  of  the  Cardiac 
Cycle. — The  old  and  valuable  diagram  reproduced  below  is 
meant  to  convey  through  the  eye  the  relations  of  the  main 
events  in  a  complete 
beat  of  the  heart  or 
cardiac  cycle.  The 
relative  length  of  the 
sounds;  the  long  peri- 
od occupied  by  the 
pause ;  the  duration  of 
the  ventricular  sys- 
tole, which  it  is  to  be 
observed  is  in  excess 
of  that  of  the  first 
sound,  are  among  the 
chief  facts  to  be  noted. 

The  tracings  of 
Chauveau  and  Marey, 
obtained      from      the 

heart      Of      the      horse,     Fig.  204.— Diagram  representing  the  movements  and 
,  .   ,    ,  ,  sounds  of  the  heart  during  a  cardiac  cycle  (after 

which  has  a  very  slow         sharpey). 


238  COMPARATIVE  PHYSIOLOGY. 

rhythm,  show  that  of  the  whole  period,  the  auricular  systole 
occupies  £  or  Tao  of  a  second  ;  the  ventricular  systole,  f  or  T\  of 
a  second ;  and  the  diastole,  f  or  T%  of  a  second. 

With  the  more  rapid  beat  in  man  (70  to  80  per  minute),  the 
duration  of  the  cardiac  cycle  may  be  estimated  at  about  ■&  of 
a  second,  and  the  probable  proportions  for  each  event  are  about 
these :  The  auricular  systole,  TV  of  a  second ;  the  ventricular 
systole,  T3o  of  a  second ;  and  the  pause,  T*o  of  a  second. 

It  will  be  noted  that  the  pause  of  the  heart  is  equal  in  dura- 
tion to  the  other  events  put  together ;  and  even  assuming  that 
there  is  some  expenditure  of  energy  in  the  return  (relaxation) 
of  the  heart  to  its  passive  form,  there  still  remains  a  consider- 
able interval  for  rest,  so  that  this  organ,  the  very  type  of  cease- 
less activity,  has  its  periods  of  complete  repose. 

THE   WORK   OF  THE   HEART. 

Since  the  pressure  against  which  the  heart  works  must,  as 
we  shall  see,  vary  from  moment  to  moment,  and  sometimes 
very  considerably,  the  work  of  the  heart  must  also  vary  within 
wide  limits,  even  making  allowance  for  large  adaptability  to 
the  burden  to  be  lifted ;  for  it  will  be  borne  in  mind  that  the  de- 
gree to  which  the  heart  empties  its  chambers  is  also  variable. 

If  one  knew  the  quantity  of  blood  ejected  by  the  left  ven. 
tricle,  and  the  rate  of  the  beat,  the  calculation  of  the  work  done 
would  be  an  easy  matter,  since  the  former  multiplied  by  the 
latter  would  represent,  as  in  the  case  of  a  skeletal  muscle,  the 
work  of  the  muscles  of  the  left  ventricle ;  from  which  the  work 
of  the  other  chambers  might  be  approximately  calculated. 

The  work  of  the  auricles  must  be  slight.  The  right  ven- 
tricle, it  is  estimated,  does  from  one  fourth  to  one  third  the 
work  of  the  left. 

When  we  calculate  the  work  done  by  the  heart  for  certain  in- 
tervals, as  the  day,  the  week,  month,  year,  and  especially  for  a 
moderate  lifetime,  and  compare  this  with  that  of  any  machine  it 
is  within  the  highest  modern  skill  to  construct,  the  great  superi- 
ority of  the  vital  pump  in  endurance  and  working  capacity  will 
be  very  apparent ;  not  to  take  into  the  account  at  all  its  wonder- 
ful adaptations  to  the  countless  vicissitudes  of  life,  without  which 
it  would  be  absolutely  useless,  even  destructive  to  the  organism. 

Some  of  these  variations  in  the  working  of  the  heart  we  may 
now  to  advantage  consider. 


THE  CIRCULATION  OF  THE  BLOOD.  239 

VARIATIONS  IN  THE   CARDIAC   PULSATION. 

These  may  be  ascertained  either  by  the  investigation  of  the 
arteries  or  of  the  heart,  for  every  considerable  alteration  in  the 
working  of  the  heart  expresses  itself  also  through  the  arterial 
system.  In  speaking  of  the  pulse,  the  reference  is  principally 
to  the  arteries,  but  in  each  case  we  may  equally  well  think  of 
the  heai-t  primarily  as  acting  upon  the  arteries. 

1.  The  frequency  of  the  heart-beat  varies,  as  might  be  sup- 
posed, with  a  great  multitude  of  conditions,  the  principal  of 
which  are :  age,  being  most  frequent  at  birth,  gradually  slow- 
ing to  old  age,  while  in  feeble  old  age  the  heart-beat  may,  like 
many  other  of  the  functions  of  the  body,  approximate  the  con- 
dition at  birth,  being  very  frequent,  small,  feeble,  and  easily 
disturbed  in  its  rhythm ;  sex,  the  cardiac  beat  being  more  fre- 
quent in  females ;  posture,  most  rapid  in  the  standing  position, 
slower  when  sitting,  and  slowest  in  the  recumbent  attitude; 
season,  more  frequent  in  summer ;  period  of  the  day,  more 
frequent  in  the  afternoon  and  evening.  Elevation  of  tem- 
perature, the  inspiratory  act,  emotions,  and  mental  activity, 
eating,  muscular  exercise,  etc.,  render  the  heart-beats  more 
frequent. 

2.  The  length  of  the  systole,  though  variable,  is  more  con- 
stant than  that  of  the  diastole. 

3.  Tlie  force  of  the  pulsation  varies  very  greatly  and  exer- 
cises an  important  influence  on  the  blood-pressure  and  the 
velocity  of  the  blood-stream.  As  a  rule,  when  the  heart  beats 
rapidly,  especially  for  any  considerable  length  of  time,  the 
force  of  the  individual  pulsations  is  diminished. 

4.  The  heart-beat  may  vary  much  and  in  ways  it  is  quite 
possible  to  estimate,  either  directly  by  the  hand  placed  over  the 
organ  on  the  chest,  by  the  modifications  of  the  cardiac  sounds, 
or  by  the  use  of  instruments.  It  is  wonderful  how  much  in- 
formation may  be  conveyed,  without  the  employment  of  any 
instruments,  through  palpation  and  auscultation,  to  one  who 
has  long  investigated  tbe  heart  and  the  arteries  with  an  intelli- 
gent, inquiring  mind;  and  we  strongly  recommend  the  student 
to  commence  personal  observations  early  and  to  maintain  them 
persistently. 

Practitioners  recognize  the  pulse  (and  heart)  as  "  slow  "  as  dis- 
tinguished from  "  infrequent,"  "  slapping,"  "  heaving,"  "  thrill- 
ing," u  bounding,"  etc. 


240 


COMPARATIVE  PHYSIOLOGY. 


Now,  if  with  these  terms  there  arise  in  the  mind  correspond- 
ing mental  pictures  of  the  action  of  the  heart  under  the  cir- 
cumstances, well  ;  if  not,  there  is  a  very  undesirable  blank. 
How  the  student  may  be  helped  to  a  knowledge  of  the  actual 
behavior  of  the  heart  under  a  variety  of  conditions  we  shall 
endeavor  to  explain  later. 

Apart  from  all  the  above  peculiarities,  the  heart  may  cease 
its  action  at  regular  intervals,  or  at  intervals  which  seem  to 
possess  no  definite  relations  to  each  other — that  is,  the  heart 
may  be  irregular  in  its  action,  which  may  be  made  evident 
either  to  the  hand  or  the  ear. 

There  are  certain  deviations  from  the  quicker  rhythm  whicb 
occur  with  such  regularity  and  are  so  dependent  on  events  that 
take  place  in  other  parts  of  the  body  that  they  may  be  con- 
sidered normal. 

Comparative. — We  strongly  recommend  the  student  to  verify 
all  the  statements  made  in  these  sections  by  direct  observation 
for  himself.  Such  is  invaluable  to  the  practitioner.  The  fol- 
lowing table  gives  the  mean  number  of  cardiac  pulsations  per 
minute  (after  Gamgee) : 


SPECIES. 


Adult. 


Horse 36-  40 

Ass  and  mule 46-  50 

Ox 45-50 

Sheep  and  goat 70-80 

Pig. 70-  80 

Dog 90-100 

Cat 120-140 


Youth. 


60-  72 

65-  75 

60-  70 

85-  95 

100-110 

110-120 

120-140 


Old  age. 


32-  38 
55-  60 
40-  45 
55-  60 
55-  60 
60-  70 
100-120 


The  variations  with  age,  for  the  horse  and  the  ox,  are  as  fol- 
lows, according  to  Kreutzer  : 

Horse.  Ox. 

At  birth 100-120 

When  14  days  old 80-96 

When  3  months  old 68-  76 

When  6  months  old 64-  72 

When  1  year  old 48-56 

When  2  years  old 40-48 

When  3  years  old 38-48 

When  4  years  old 38-50 

When  aged 32-  40 


At  birth 92-132 

When  4-5  days  old 100-120 

When  14  days  old 68 

When  4-6  weeks  old 64 

When  6-12  months  old  . .  56-  68 

For  the  young  cow 46 

For  the  four-year-old  ox  .  40 


THE   CIRCULATION   OF  THE  BLOOD. 


241 


THE  PULSE. 

Naturally  the  intermittent  action  of  the  heart  gives  rise  to 
corresponding  phenomena  in  the  elastic  tubes  into  which  it 
may  he  said  to  be  continued,  for  it  is  very  desirable  to  keep  in 
mind  tbe  complete  continuity  of  the  vascular  system. 

The  following  phenomena  are  easy  of  observation  :  When 
a  finger- tip  is  laid  on  any  artery,  an  interrupted  pressure  is  felt ; 
if  the  vessel  be  laid  bare  (or  observed  in  an  old  man),  it  may 
be  seen  to  be  moved  in  its  bed  forward  and  upward  ;  the  press- 
ure is  less  the  farther  the  artery  from  the  heart ;  if  the  vessel 
be  opened,  blood  flows  from  it  continuously,  but  in  spurts  ;  if 
one  finger  be  laid  on  the  carotid  and  another  on  a  distant  ves- 
sel, as  one  of  the  arteries  of  the  foot,  it  may  be  observed  (though 
it  is  not  easy  from  difficulty  in  attending  to  two  events  hap- 
pening so  very  close  together)  that  the  beat  in  the  nearer  ves- 
sel precedes  by  a  slight  interval  that  in  the  more  distant. 

Investigating  the  latter  phenomenon  with  instruments,  it  is 
found  tbat  an  appreciable  interval,  depending  on  the  distance 
apart  of  the  points  observed,  intervenes. 

What  is  the  explanation  of  these  facts  ? 

The  student  may  get  at  this  by  a  few  additional  observa- 
tions that  can  be  easily  made. 


Fig.  205. — Marey's  apparatus  for  showing  the  mode  in  which  the  pulse  is  propagated 
in  the  arteries.  B.  a  rubber  pump,  with  valves  to  prevent  regurgitation.  The 
working  of  the  apparatus  will  be  apparent  from  the  inspection  of  the  figure. 

If  water  be  sent  through  a  long  elastic  tube  (so  coiled  that 
points  near  and  remote  may  be  felt  at  the  same  time)  by  a  bulb 

1(3 


242  COMPARATIVE  PHYSIOLOGY. 

syringe,  imitating  the  heart,  and  against  a  resistance  made  by 
drawing  out  a  glass  tube  to  a  fine  point  and  inserting  it  into 
the  terminal  end  of  the  rubber  tube,  an  intermittent  pi'essure 
like  that  occurring  in  the  artery  may  be  observed ;  and  further 
that  it  does  not  occur  at  precisely  the  same  moment  at  the  two 
points  tested. 

Information  more  exact,  though  possibly  open  to  error,  may 
be  obtained  by  the  use  of  more  elaborate  apparatus  and  the 
graphic  method. 

By  measurement  it  has  been  ascertained  that  in  man  the 
pulse-wave  travels  at  the  rate  of  from  five  to  ten  metres  per  sec- 
ond, being  of  course  very  variable  in  velocity.  It  would  seem 
that  the  more  rigid  the  arteries  the  more  rapid  the  rate,  for  in 
children  with  their  more  elastic  arteries  the  speed  is  slower ; 
and  the  same  principle  is  supposed  to  explain  the  higher  veloci- 
ty noticed  in  the  arteries  of  the  lower  extremities.  But  with 
such  a  speed  as  even  five  metres  a  second  it  is  evident  that  with 
a  systole  of  moderate  duration  (say  '3  second)  the  most  distant 
arteriole  will  have  been  reached  by  the  pulse-wave  before  that 
systole  is  completed. 

It  is  known  that  the  blood-current  at  its  swiftest  has  no 
such  speed  as  this,  never  perhaps  exceeding  in  man  half  a  metre 
per  second,  so  that  the  pulse  and  the  blood-current  must  be  two 
totally  distinct  things. 

When  the  left  ventricle  throws  its  blood  into  vessels  already 
full  to  distention,  there  must  be  considerable  concussion  in  con- 
sequence of  the  rapid  and  forcible  nature  of  the  cardiac  systole, 
and  this  gives  rise  to  a  wave  in  the  blood  which,  as  it  passes 
along  its  surface,  causes  each  part  of  every  artery  in  succession 
to  respond  by  an  elevation  above  the  general  level,  and  it  is  this 
which  the  finger  feels  when  laid  upon  an  artery. 

That  there  is  considerable  distention  of  the  arterial  system 
with  each  pulse  may  be  realized  in  various  ways,  as  by  watch- 
ing and  feeling  an  artery  laid  bare  in  its  course,  or  in  very  thin 
or  very  old  people,  and  by  noticing  the  jerking  of  one  leg 
crossed  over  the  other,  by  which  method  in  fact  the  pulse-rate 
may  be  ascertained.  And  that  not  only  the  whole  body  but 
the  entire  room  in  which  a  person  sits  is  thrown  into  vibra- 
tion by  the  heart's  beat,  may  be  learned  by  the  use  of  a  tele- 
scope to  observe  objects  in  the  room,  which  may  thus  be  seen  to 
be  in  motion. 

Features  of  an  Arterial  Pulse-Tracing.— In  order  to  judge  of 


THE   CIRCULATION   OF   THE  BLOOD. 


243 


the  nature  of  arterial  tracings,  it  is  important  that  the  circum- 
stances under  which  they  are  obtained  should  be  known. 


;.  »uo. — luarey  s  improveu  spnygmograpn  arranged  lor  iaK 

spring;  B,  first  lever;  C,  writing  lever;  6",  its  free  writing  end;  D\  screw  for 
bringing  B  in  contact  with  C\  G,  slide  with  smoked  paper;  H,  clock-work;  L, 
screw  for  increasing  the  pressure;  M,  dial  indicating  the  amount  of  pressure, 
K,  A",  straps  for  fixing  the  instrument  to  the  arm,  and  the  latter  to  the  double- 
inclined  plane  or  support  (Byrom  Bramwell). 


The  movements  of  the  vessel  wall  in  most  mammals  suitable 
for  experiment  aud  in  man  is  so  slight  that  it  becomes  necessary 
to  exaggerate  them  in  the  tracing,  hence  long  levers  are  used  to 
accomplish  this. 

The  sphygmograph  is  the  usual  form  of  instrument  em- 
ployed for  the  purpose.  It  consists,  essentially,  of  a  clock-work 
for  moving  a  smoked  surface 
(mica  plate  commonly)  on 
which  the  movements  of  a 
lever-tip,  answering  to  those 
of  a  button  placed  on  the 
artery,  are  recorded. 

We  shall  do  well  to  in- 
quire whether  there  are  any 
features  in  common  in  trac- 
ings obtained  in  various 
ways,  and  which  have  there- 
fore in  all  probability  a  real  foundation  in  nature. 

An  inspection  of  a  large  number  of  pulse- tracings,  taken  un- 
der diverse  conditions,  seems  to  show  that  in  all  of  them  there 
occurs,  more  or  less  marked,  the  following  :     1.     An  upward 


Fig.  207.— Diagrammatic  schema  showing  the 
essential  part  of  the  instrument  when  in 
working  order.  The  knife-edge  B"  of 
the  short  lever  is  in  contact  with  the 
writing-lever  C.  Every  movement  of  the 
steel  spring  at  A",  communicated  by  the 
arteries,  will  be  imparted  to  the  writing- 
lever  (Byrom  Bramwell). 


244 


COMPARATIVE  PHYSIOLOGY. 


curve.     2.  A  downward  curve,  rendered  irregular  by  the  occur- 
rence of  peaks  or  crests  and  notches.     The  first  of  these  are 


Fig,  208.— Pulse  tracing  from  carotid  artery  of  healthy  man  (after  Moens).  x,  com- 
mencement of  expansion  of  artery;  A,  summit  of  first  rise;  C,  dicrotic  secondary 
wave;  B,  predicrotic  secondary  wave;  p,  notch  preceding  this;  D,  succeeding  sec- 
ondary wave.  Curve  above  is  that  made  by  a  tuning-fork  with  ten  double  vibra- 
tions in  a  second. 

termed  the  predicrotic  notch  and  crest,  and  the  succeeding  ones 
the  dicrotic  notch  and  crest.  The  latter  seem  to  be  the  more 
constant. 

Venous  Pulse. — Apart  from  the  variations  in  the  caliber  of 
the  great  veins  near  the  heart,  constituting  a  sort  of  pulse, 
though  due  to  variations  in  intra-cardiac  pressure,  a  venous 
pulse  proper  is  rare  as  a  normal  feature.  One  of  the  best-known 
examples  of  such  occurs  in  the  salivary  gland.  When,  during 
secretion,  the  arterioles  are  greatly  dilated,  a  pulse  may  be  wit- 
nessed in  the  veins  into  which  the  capillaries  open  out,  owing 
to  diminution  in  the  resistance  which  usually  is  sufficiently 
great  to  obliterate  the  pulse-wave. 

Pathological. — In  severe  cases  of  heart-disease,  owing  to 
cardiac  dilatation  or  other  conditions,  giving  rise  to  incompe- 
tency of  the  tricuspid  valves,  there  may  be  with  each  ventricu- 
lar systole  a  back-flow,  visible  in  the  veins  of  the  neck. 

A  venous  pulse  is  a  phenomenon,  it  will  be  evident,  that 
always  demands  special  investigation.  It  means  that  the  usual 
bounds  of  nature  are  for  some  good  reason  being  overstepped. 

Comparative. — Before  entering  on  the  consideration  of  phe- 
nomena that  all  are  agreed  are  purely  vital,  we  call  attention  to 
the  circulation  in  forms  lower  than  the  mammal,  in  order  to 
give  breadth  to  the  student's  views  and  prepare  him  for  the 
special  investigations,  which  must  be  referred  to  in  subsequent 
chapters;  and  which,  owing  to  the  previous  narrow  limits  (re- 
searches upon  the  frog  and  a  few  well-known  mammals)  having 
at  last  been  overleaped,  have  opened  up  entirely  new  aspects  of 


THE  CIRCULATION  OP  THE   BLOOD.  215 

cardiac  physiology — one  might  almost  say  revolutionized  the 
subject. 

Owing  to  the  limitations  of  our  space,  the  references  to  lower 
forms  must  be  brief. 

We  recommend  the  student,  however,  to  push  the  subject 
further,  and  especially  to  carry  out  some  of  the  experiments  to 
which  attention  will  be  directed  very  shortly. 

In  the  lowest  organisms  {Infusorians)  represented  by  Amoe- 
ba, Vorticella,  etc.,  there  are,  of  course,  no  circulatory  organs, 
unless  the  pulsating  vacuoles  of  some  forms  mark  the  crude 
beginnings  of  a  heart.  It  will  be  borne  in  mind,  bowever,  that 
there  is  a  constant  streaming  of  the  protoplasm  itself  within 
the  organism. 

The  heart  is  first  represented,  as  in  worms,  by  a  pulsatile 
tube,  which  may,  as  in  the  earth-worm,  extend  throughout  the 
greater  part  of  the  length  of  the  animal,  and  has  usually  dorsal 
and  ventral  and  transverse  connections 

The  dilatations  of  the  transverse  portions  in  one  division 
(metamere)  of  the  animal  seem  to  foreshadow  the  appearance  of 
auricles. 

The  pulsation  of  the  dorsal  vessel  in  a  large  earth-worm  is 
easy  of  observation. 

In  amphioxus,  which  is  often  instanced  as  the  lowest  verte- 
brate, the  blood-vessels,  including  the  portal  vein,  are  pulsatile, 
while  there  is  no  distinct  and  separate  heart. 

Although  the  respiratory  system  will  be  treated  from  the 
comparative  point  of  view,  the  student  will  do  well  to  note  now 

Ab  Ao  4' 


Ba     V         ^7F 

Fig.  209.— Diagram  of  the  circulation  of  a  Teleostean  fish  (Clans).  V.  ventricle;  Ba, 
bulbus  arteriosus,  with  the  arterial  arches  which  carry  the  blood  to  the  gills;  Ab, 
arterial  arches:  Jo,  aorta  descendens,  into  which  the  epibranchiaJ  arteries  passing 
out  from  the  gills  unite;  K,  kidneys;  /,  intestine;  Pc.  portal  circulation. 

(in  the  figures)  the  close  relation  between  the  organs  for  dis- 
tributing and  aerating  the  blood. 

Passing  on  to  the  vertebrates,  in  the  lowest  group,  the  fishes, 


246 


COMPARATIVE   PHYSIOLOGY. 


the  heart  consists  of  two  chambers,  an  auricle  and  a  ventricle, 
the  latter  being  supplemented  by  an  extension  (bulbus  arterio- 
sus) pulsatile  in  certain  species  ;  and  an  examination  of  the 


Fig.  210. 


Pig.  211. 


Fig.  210. — The  arterial  trunks  and  their  main  branches  in  the  frog  {Rana  esculenta). 
1  x  1|.  (Howes.)  I,  lingual  vessel;  c.  c,  common  carotid  artery;  p.cu,  pulmo- 
cutaneous  artery;  c.  gl,  carotid  gland;  aw',  right  auricle;  aw",  left  auricle;  v,  ven- 
tricle; tr.a,  truncus  arteriosus;  pul',  pulmonary;  Ig,  left  lung;  ao,  left  aortic 
arch;  br,  brachial;  cu)  cutaneous;  d.  ao,  dorsal  aorta;  cm,  coeliaco-mesenteric; 
cm',  coeliac;  hn,  hepatic  vessels;  17,  gastric;  j>c',  pancreas;  m,  mesenteric;  sp, 
splenic;  du',  duodenal;  h,  hajmorrhoidal;  W,  ileal;  h,y,  hypogastric;  c.  II,  com- 
mon iliac;  re,  renal;  k,  kidney;  Is,  spermatic. 

Fig.  211.— Venous  trunks  and  their  main  branches  in  the  frog  (Rana  esculenta).  1  x  1J. 
(Howes.)  I,  lingual  vein;  e.j,  external  jugular;  in,  innominate;  i.j,  internal  jugu- 
lar; s.  sc,  subscapular;  pr.  c,  vena  cava  superior;  s.  v,  sinus  venosus;  hp,  hepatic; 
I11',  right  lobe  of  liver;  Iv",  left  lobe  of  liver;  pt.  c,  vena  cava  inferior;  ov,  ovarian; 
d.  I,  dorso-lumbar;  od,  oviducal;  r.p,  renal-portal;  fm,  femoral;  sc,  sciatic;  a, 
femoro-sciatic  anastomosis;  pv' ,  right  pelvic;  vs,  vesical;  ant.  ab,  anterior  ab- 
dominal; a',  abdominal-portal  anastomosis;  W,  ileal;  sp,  splenic;  du',  duodenal; 
I.  int,  lieno-intestinal;  g,  gastric;  p,  portal;  Ig",  left  lung;  pul,  pulmonary;  m.  cu, 
musculo-cutancous;  br,  brachial. 


course  of  the  circulation  will  show  that  the  heart  is  throughout 
venous,  the  blood  being  oxidized  in  the  gills  after  leaving  the 
former. 

Among  the  amphibians,  represented  by  the  frog,  there  are 
two  auricles  separated  by  an  almost  complete  septum,  and  one 


THE   CIRCULATION   OF   THE  BLOOD. 


247 


ventricle  characterized  by  a  spongy  arrangement  of  the  muscle- 
fibers  of  its  walls. 

In  the  reptiles  the  division  between  the  auricles  is  complete, 
and  there  is  one  ventricle  which  shows  imperfect  subdivisions. 


Fig.  212. 


Fig  213. 


Fig.  212.— The  frog's  heart,  seen  from  the  front,  the  aortic  arches  of  the  left  side  hav- 
ing been  removed.  (1  x  4.)  ca,  carotid;  c.  ffl,  carotid  gland;  ao,  aorta;  an',  right 
auricle;  au",  left  auricle;  pr.  c,  vena  cava  superior;  pt.  c,  vena  cava  inferior; 
p.  cu,  pulmo-cutaneous  trunk;  tr,  truncus  arteriosus:  v,  ventricle  (Howes). 

Fig.  213.— The  same,  seen  from  behind,  the  sinus  venosus  having  been  opened  up  10 
show  the  sinu-auricular  valves.  (1  x  4.)  p.v,  pulmonary  vein ;  s.  v,  sinus  veno- 
bus;  va",  sinu-auricular  valve.    Other  lettering  as  in  Fig.  212  (Howes). 

In  the  crocodile,  however,  the  heart  consists  of  four  per- 
fectly divided  chambers.  Of  the  two  aortic  arches,  one  arises 
together  with  the  pulmonary  artery  from  the  right  ventricle, 
and,  as  it  crosses  over,  the  left  communicates  with  it  by  a  small 
opening,  so  that,  although  the  arterial  and  the  venous  blood 
are  completely  separated  in  the  heart,  they  intermingle  outside 
of  this  organ. 

In  birds  the  circulatory  system  is  substantially  the  same  as 
in  mammals  ;  but  in  all  vertebrate  forms  below  birds  the  blood 
distributed  to  the  tissues  is  imperfectly  oxidized  or  is  partially 
venous. 

As  a  result  of  the  entire  vascular  arrangements  in  the  frog, 
etc.,  the  least  oxidized  blood  passes  to  the  lungs,  and  the  most 
aerated  to  the  head  and  anterior  parts  of  the  animal. 

Whatever  ground  for  differences  of  opinion  there  may  be 
as  to  the  extent  to  which  the  phenomena  we  have  as  yet  been 
describing  are  mechanical  in  their  nature,  all  are  agreed  that 


24S  COMPARATIVE  PHYSIOLOGY. 

such  explanations  are  insufficient  when  applied  to  the  facts 
with  which  we  have  yet  to  deal.  They,  at  all  events,  can  be 
regarded  only  as  the  result  of  vitality. 

When  one  reflects  upon  the  vicissitudes  through  which  an 
animal  must  pass  daily  and  hourly,  necessitating  either  that 
they  be  met  by  modified  action  of  the  organs  of  the  body  or 
that  the  destruction  of  the  organism  ensue,  it  becomes  clear  that 
the  varying  nutritive  needs  of  each  part  must  be  answered  by 
changes  in  the  circulatory  system.  These  changes  may  affect 
any  part  of  the  entire  arrangement,  and  it  rarely  happens,  as 
will  appear,  that  one  part  is  modified  without  a  corresponding 
one,  very  frequently  of  a  different  kind,  taking  place  in  some 
other.  What  these  various  correlated  modifications  are,  and 
how  they  are  brought  about,  we  shall  now  attempt  to  describe, 
and  it  will  greatly  assist  in  the  comprehension  of  the  whole  if 
the  student  will  endeavor  to  keep  a  clear  mental  picture  of  the 
parts  before  his  mind  throughout,  using  the  figures  and  verbal 
descriptions  only  to  assist  in  the  construction  of  such  a  mental 
image.     We  shall  begin  with  the  vital  pump — the  heart. 

THE  BEAT  OF  THE   HEART  AND  ITS  MODIFICATIONS. 

As  has  been  already  noted,  the  cardiac  muscle  has  features 
peculiar  to  itself,  and  occupies  histologically  an  intermediate 
place  between  the  plain  and  the  striped  muscle-cells,  and  that 
the  contraction  of  the  heart  is  also  intermediate  in  character, 
and  is  best  seen  in  those  fox*ms  of  the  organ  which  are  somewhat 
tubular  and  beat  slowly.  But  the  contraction,  though  peristal- 
tic, is  more  rapid  than  is  usually  the  case  in  organs  with  the 
smooth  form  of  muscle-fiber. 

The  heart  behaves  under  a  stimulus  in  a  peculiar  manner, 
the  effect  of  a  single  induction  shock  depends  on  the  phase  of 
contraction  hi  which  the  heart  happens  to  be  at  the  moment  of 
its  application.  Thus  at  the  commencement  of  a  systole  there 
is  no  visible  effect,  while  beats  of  unusual  character  result  at 
other  times.  But  tetanus  can  not  be  induced  by  any  form  or 
method  of  stimulation.  The  latent  period  of  cardiac  muscle  is 
long. 

In  a  heart  at  rest  a  single  stimulus  (as  the  prick  of  a  needle) 
usually  calls  forth  but  one  contraction. 


THE  CIRCULATION  OF  THE  BLOOD.  249 


THE  NERVOUS  SYSTEM  IN  RELATION  TO  THE  HEART. 

The  attempts  to  determine  just  why  the  heart  heats  at  all, 
and  especially  the  share  taken  by  the  nervous  system,  if  any 
direct  one,  are  beset  with  great  difficulty  ;  though,  as  we  shall 
attempt  to  show  later,  this  subject  also  has  been  cramped  within 
too  narrow  limits,  and  hence  regarded  in  a  false  light. 

Till  comparatively  recently  the  frog's  heart  alone  received 
much  attention,  if  we  except  those  of  certain  well-known  mam- 
mals. In  the  heart  of  the  frog  there  are  ganglion-cells  in  vari- 
ous parts,  especially  numerous  in  the  sinus  venosus  (or  expan- 
sion of  the  great  veins  where  they  meet  the  auricles) ;  also  in  the 
auricles,  more  especially  in  the  septum  (ganglia  of  Remak),  while 
they  are  absent  from  the  greater  part  of  the  ventricle,  though 
found  in  the  auriculo-ventricular  groove  (ganglia  of  Bidder). 

Recently  it  has  been  found  that  ganglion-cells  occur  in  the 
ventricles  of  warm-blooded  animals.  In  the  hearts  of  the  dog, 
calf,  sheep,  and  pig,  which  are  those  lately  subjected  to  investi- 
gation, it  is  found  that  the  nerve-cells  do  not  occur  near  the 
apex  of  the  ventricles,  but  mainly  in  the  middle  and  basal  por- 
tions, being  most  abundant  in  the  anterior  and  posterior  inter- 
ventricular furrows  and  in  the  left  ventricle.  But  there  are 
differences  for  each  group  of  animals  ;  thus,  these  ganglion- 
cells  are  most  abundant,  so  far  as  the  mammals  as  yet  inves- 
tigated ai'e  concerned,  in  the  ventricles  of  the  pig,  and  least  so 
in  those  of  the  dog.  In  tbe  cat  they  ai'e  also  scanty.  Ganglion- 
cells  occur  in  the  auricles,  and  are  especially  abundant  near  the 
terminations  of  the  great  veins. 

It  has  long  been  known  that  the  heart  of  a  frog  removed 
from  the  body  will  pulsate  for  hours,  especially  if  fed  with 
serum,  blood,  or  similar  fluids  ;  and  that  it  may  be  divided  in 
almost  any  conceivable  way,  even  when  teased  up  into  minute 
particles,  and  still  continue  to  beat.  The  apex,  however,  when 
separated  does  not  beat.  Yet  even  this  quiescent  apex  may  be 
set  pulsating  if  tied  upon  the  end  of  a  tube,  through  which  it 
may  be  fed  under  pressure. 

We  may  here  point  out  that  the  whole  heart  or  a  part  of  it 
may  be  made  to  describe  its  action  by  the  graphic  method  in 
various  ways,  the  principles  underlying  which  are  either  that 
the  heart  pulls  upon  a  recording  lever  (lifts  it) ;  acts  against  the 
fluid  of  a  manometer  ;  or,  inclosed  in  a  vessel  containing  oil  or 
similar  fluid,  moves  a  piston  in  a  cylinder. 


250  COMPARATIVE   PHYSIOLOGY. 

It  lias  also  long  been  known  that  a  ligature  drawn  around 
the  sinus  venosus  (in  the  frog)  at  its  junction  with  the  auricles 
stopped  the  heart  for  a  certain  period,  and  this  experiment  (of 
Stannius)  was  thought  to  demonstrate  that  the  heart  was  ar- 
rested because  the  nervous  impulses  proceeding  to  the  ganglion- 
cells  along  the  cardiac  nerves  or  ganglia  of  this  region  were 
cut  off  by  the  ligature  ;  in  other  words,  the  heart  ceased  to  beat 
because  the  outside  machinery  on  which  the  action  of  the  inner 
depended  was  suddenly  disconnected.  Other  explanations  have 
been  offered  of  this  fact. 

Within  the  last  few  years  great  light  has  been  thrown  upon 
the  whole  subject  of  cardiac  physiology  in  consequence  of  in- 
vestigators having  studied  the  hearts  of  various  cold-blooded 
animals  and  of  several  invertebrates.  The  hearts  of  the  Che- 
lonians  (tortoises,  turtles)  have  received  special  attention,  and 
their  investigation  has  been  fruitful  of  results,  to  the  general 
outcome  of  which,  as  well  as  those  accruing  from  recent  com- 
parative studies  as  a  whole,  we  can  alone  refer.  Since  in  other 
parts  of  the  work  the  limits  of  space  will  not  always  allow  us 
to  give  the  evidence  on  which  conclusions  rest,  attention  is 
especially  called  to  what  here  follows,  as  an  example  of  the 
methods  of  physiological  research,  and  the  nature  of  the  reason- 
ing employed. 

Very  briefly  the  following  are  some  of  the  main  facts  : 

1.  In  all  cold-blooded  animals  the  order  in  which  the  sub- 
divisions of  the  heart  ceases  to  pulsate  when  kept  under  the 
same  conditions  is  invariable,  viz.,  ventricle,  auricles,  sinus. 

2.  The  sinus  and  auricles,  when  separated  by  section,  liga- 
ture, or  otherwise,  either  together  or  singly,  continue  to  beat, 
whether  amply  provided  with  or  surrounded  by  blood. 

3.  The  ventricle  thus  separated  displays  less  tendency  to 
beat  independent  of  some  stimulus  (as  feeding  under  pressure), 
though  a  very  weak  one  usually  suffices — i.  e.,  its  tendency  to 
spontaneous  rhythm  is  less  marked  than  is  the  case  with  the 
other  parts  of  the  heart.  These  remarks  apply  to  the  hearts 
of  Chelonians — fishes,  snakes,  and  some  other  cold-blooded 
animals. 

4.  In  certain  fishes  (skate,  ray,  shark)  the  beat  may  be  re- 
versed by  stimulation,  as  a  prick  of  the  ventricle.  This  is 
accomplished  with  more  difficulty  in  other  cold-blooded  ani- 
mals, and  still  more  so  in  the  mammal. 

5.  In  certain  invertebrates,  notably  the  Poulpe  (Octopus),  a 


THE  CIRCULATION   OF   THE   BLOOD.  251 

careful  search  has  revealed  no  nerve-cells,  yet  their  hearts  con- 
tinue to  beat  when  their  nerves  are  severed,  on  section  of  parts 
of  the  organ,  etc. 

6.  A  strip  of  the  muscle  from  the  ventricle  of  the  tortoise, 
when  placed  in  a  moist  chamber  and  a  current  of  electricity 
passed  through  it  for  some  hours,  will  commence  to  pulsate  and 
continue  to  do  so  after  the  current  has  been  withdrawn  ;  and 
this  holds  when  the  strip  is  wholly  free  from  nerve-cells. 

From  the  above  facts  certain  inferences  have  been  drawn  : 
1.  It  has  been  concluded  that  the  sinus  is  the  originator  and 
director  of  the  movements  of  the  rest  of  the  heart.  2.  That  this 
is  owing  to  the  ganglia  in  its  walls.  While  all  recognize  the 
importance  of  the  sinus,  some  physiologists  hold  to  the  gangli- 
onic influence  as  essential  to  the  heart-beat  still  ;  while  others, 
influenced  by  the  facts  mentioned  above,  are  disposed  to  regard 
tbem  as  of  very  doubtful  importance — at  all  events,  as  origina- 
tors of  the  movements  of  the  heart. 

The  tendency  now  seems  to  be  to  attach  undue  importance 
to  the  spontaneous  contractility  of  the  heart-muscle  ;  for  it  by 
no  means  follows  logically  that,  because  a  muscle  treated  by 
electricity,  when  cut  off  from  the  usual  nerve  influence  that  we 
believe  is  being  constantly  exerted  on  the  heart  like  other  or- 
gans, will  contract  and  continue  to  do  so  in  the  absence  of  the 
stimulus,  it  does  so  normally.;  or,  because  some  hearts  beat  in 
the  absence  of  nerve-cells,  that  therefore  nerve-cells  are  of  no 
account  in  any  case.  Such  views,  when  pressed  to  the  extreme, 
lead  to  as  narrow  conceptions  as  those  they  are  intended  to  re- 
place. 

Taking  into  account  the  facts  mentioned  and  others  we  have 
not  space  to  enumerate,  we  submit  the  following  as  a  safe  view 
to  entertain  of  the  beat  of  the  heart  in  the  light  of  our  present 
knowledge  : 

Recent  investigations  show  clearly  that  there  are  great  dif- 
ferences in  the  hearts  of  animals  of  diverse  groups,  so  that  it 
is  not  possible  to  speak  of  "the  heart"  as  though  our  remarks 
applied  equally  to  this  organ  in  all  groups  of  animals. 

It  must  be  admitted  that  our  understanding  of  the  hearts  of 
the  cold-blooded  animals  is  greater  than  of  the  mammalian 
heart ;  while,  so  far  as  exact  or  experimental  knowledge  is  con- 
cerned, the  human  heart  is  the  least  understood  of  all,  though 
there  is  evidence  of  a  pathological  and  clinical  kind  and  subject- 
ive experience  on  which  to  base  conclusions  possessing  a  certain 


252  COMPARATIVE  PHYSIOLOGY. 

value ;  but  it  is  clear  to  those  who  have  devoted  attention  to 
comparative  physiology  that  the  more  this  subject  is  extended 
the  better  prepared  we  shall  be  for  taking  a  broad  and  sound 
view  of  the  physiology  of  the  human  heart  and  man's  other 
organs. 

Whatever  may  be  said  of  the  invertebrates,  among  which 
greater  simplicity  of  mechanism  doubtless  prevails,  there  can 
be  no  doubt  that  the  execution  of  a  cardiac  cycle  of  the  heart 
in  all  vertebrates,  and  especially  in  the  higher,  is  a  very  com- 
plex process  from  the  number  of  the  factors  involved,  their  in- 
teraction, and  their  normal  variation  with  circumstances  ;  and 
we  must  therefore  be  suspicious  of  any  theory  of  excessive  sim- 
plicity in  this  as  well  as  other  parts  of  physiology. 

We  submit,  then,  the  following  as  a  safe  provisional  view  of 
the  causation  of  the  heart-beat : 

1.  The  factors  entering  into  the  causation  of  the  heart-beat 
of  all  vertebrates  as  yet  examined  are  :  (a)  A  tendency  to  spon- 
taneous contraction  of  the  muscle-cells  composing  the  organ: 
(6)  intra-carcliac  blood-pressure ;  (c)  condition  of  nutrition  as 
determined  directly  by  the  nervous  supply  of  the  organ  and  in- 
directly by  the  blood. 

2.  The  tendency  to  spontaneous  contraction  of  muscle-cells 
is  most  marked  in  the  oldest  parts  of  the  heart  (e.g.,  sinus), 
ancestrally  (phylogenetically)  considered. 

3.  Inti*a-carcliac  pressure  exercises  an  influence  in  determin- 
ing the  origin  of  pulsation  in  probably  all  hearts,  though  like 
other  factors  its  influence  varies  with  the  animal  group.  In 
the  mollusk  (and  allied  forms)  and  in  the  fish  it  seems  to  be  the 
controlling  factor. 

4.  We  must  recognize  the  power  one  cell  has  to  excite,  when 
in  action,  neighboring  heart-cells  \o  contraction.  The  ability 
that  one  protoplasmic  cell-mass  has  to  initiate  in  others,  under 
certain  circumstances,  like  conditions  with  its  own,  is  worthy 
of  more  serious  consideration  in  health  and  disease  than  it  has 
yet  received. 

5.  The  influence  of  the  cardiac  nerves  becomes  more  pro- 
nounced as  we  ascend  the  animal  scale.  Their  share  in  the 
heart's  beat  will  be  considered  later. 

6.  Apparently  in  all  hearts  there  is  a  functional  connection 
leading  to  a  regular  sequence  of  beat  in  the  different  parts,  in 
which  the  sinus  or  its  representatives  (the  terminations  of  great 
veins  in  the  heart)  always  takes  the  initiative.    One  part  having 


THE  CIRCULATION  OF   THE   BLOOD. 


253 


contracted,  the  others  must  necessarily  follow ;  hence  the  rapid 
onset  of  the  ventricular  after  the  auricular  contraction  in  the 
mammal,  and  the  long  wave  of  contraction  that  seems  to  pass 
evenly  over  the  whole  organ  in  cold-blooded  animals. 

The  basis  of  all  these  factors  is  to  be  sought  finally  in  the 
natural  contractility  of  protoplasm.  A  heart  in  its  most  de- 
veloped form  still  retains,  so  to  speak,  the  inherited  but  modified 
Amoeba  in  its  every  cell. 

Whether  the  intrinsic  nerve-cells  of  the  heart  take  any  share 
directly  in  the  cardiac  beat  must  be  considered  as  yet  undeter- 
mined. Possibly  they  do  modify  motor  impulses  from  nerves, 
while  again  it  may  be  that  they  have  an  influence  over  nutri- 
tive processes  only.  The  subject  requires  further  study,  both 
anatomical  and  physiological. 

INFLUENCE  OF  THE  VAGUS  NERVE  UPON  THE  HEART. 

The  principal  facts  in  this  connection  may  be  stated  as  fol- 
lows, and  apply  to  all  the  animals  thus  far  examined  : 

1.  In  all  cases  the  action  of  the  heart  is  modified  by  stimu- 
lation of  the  medulla  oblongata  or  the  vagus  nerve. 

2.  The  modification  may  consist  in  prompt  arrest  of  the 
heart,  in  slowing,  in  enfeeblement  of  the  beat,  or  a  combination 
of  the  two  latter  effects. 

3.  After  the  application  of  the  stimulation  there  is  a  latent 


Fig.  214.— Inhibition  of  frog's  heart  by  stimulation  of  the  vagus  nerve.  To  be  read 
from  right  to  left.  The  contractions  of  the  ventricle  are  registered  by  a  simple 
lever  resting  on  it.  The  interrupted  current  was  thrown  in  at  a.  Note  that  one 
beat  occurred  before  arrest  (latent  period),  and  that  when  standstill  of  the  heart 
did  take  place  it  lasted  for  a  considerable  period  (.Foster). 

period  before  the  effect  is  manifest,  and  the  latter  may  outlast 
the  stimulation  by  a  considerable  period. 


254  COMPARATIVE  PHYSIOLOGY. 

4.  In  most  animals  the  sinus  venosus  and  auricles  are  af- 
fected before  the  ventricles,  and  the  vagus  may  influence  these 
parts  when  it  is  powerless  over  the  ventricle. 

5.  After  vagus  inhibition,  the  action  of  the  heart  is  (almost 
unexceptionally)  different,  the  precise  result  being  variable,  but 
generally  the  beat  is  both  accelerated  and  increased  in  force. 
We  may  say  that  the  werking  capacity  of  the  heart  is  tem- 
porarily increased. 

6.  The  improvement  in  the  efficiency  of  the  heart  is  in  pro- 
portion to  its  previous  working  power,  and  in  cases  when  the 


Stimulation  Vagus. 


Fig.  915.— Effects  of  vagus  stimulation,  illustrated  by  a  form  of  sphygmograpliic  curve 
derived  from  the  carotid  of  a  rabbit  (Foster). 

action  is  feeble  and  irregular  (abnormal)  it  might  be  said  to  be 
in  proportion  to  its  needs.  This  is  a  very  important  law  that 
deserves  to  receive  a  general  recognition. 

7.  Section  of  both  vagi  nerves  results  in  histological  altera- 
tions in  the  heart's  structure,  chiefly  fatty  degeneration,  which 
must,  of  course,  impair  its  working  capacity  and  expose  it 
to  rupture  or  other  accidents  under  the  frequently  recurring 
strains  of  life. 

8.  In  the  cold-blooded  animals  the  heart  may  be  kept  at  a 
standstill  by  vagus  stimulation  till  it  dies,  a  period  of  hours 
(one  case  of  six  hours  reported  for  the  sea-turtle). 

9.  Certain  drugs  (as  atropine),  applied  directly  to  the  heart, 
or  injected  into  the  blood,  prevent  the  usual  action  of  the  vagus. 

10.  During  vagus  arrest  the  heart  substance  undergoes  a 
change,  resulting  in  an  unusual  dilatation  of  the  organ.  This 
may  be  witnessed  whether  the  heart  contains  blood  or  not. 


THE  CIRCULATION  OP  THE  BLOOD.  255 

11.  The  heart  may  he  arrested  by  direct  stimulation,  espe- 
cially of  the  sinus,  and  at  the  points  at  which  the  electrodes  are 
applied  there  is  apparently  a  temporary  paralysis.  The  same 
alteration  in  the  beat  may  he  noticed  as  when  the  main  trunk 
of  the  vagus  is  stimulated. 

12.  The  heart  may  he  inhibited  through  stimulation  of  vari- 
ous parts  of  the  body,  both  of  the  surface  and  internal  organs 
(reflex  inhibition). 

13.  One  vagus  being  divided,  stimulation  of  its  upper  end 
may  cause  arrest  of  the  heart. 

14.  Stimulation  of  a  small  part  of  the  medulla  oblongata 
will  produce  the  same  result,  provided  one  or  both  vagi  be 
intact. 

15.  Section  of  both  vagi  in  some  animals  (the  dog  notably) 
increases  the  rate  of  the  cardiac  beat.  The  result  of  section  of 
one  pneumogastric  nerve  is  variable.  The  heart's  rhythm  is 
usually  to  some  extent  quickened. 

16.  During  vagus  inhibition  from  any  cause  in  mammals 
and  many  other  animals,  the  heart  responds  to  a  single  stimu- 
lus, as  the  prick  of  a  needle,  by  at  least  one  beat.  An  observer 
studying  for  himself  the  behavior  of  the  heart  in  several  groups 
of  animals  with  an  open  mind,  for  the  purpose  of  observing  all 
he  can  rather  than  proving  or  disproving  some  one  point,  be- 
comes strongly  impressed  with  the  variety  in  unity  that  runs 
through  cardiac  physiology,  including  the  influence  of  nerve- 
cells  (centers)  through  nerves  ;  for  it  will  not  be  forgotten  that 
normally  nerves  originate  nothing,  being  conductors  only,  so 
that  when  the  vagus  is  stimulated  by  us  we  are  at  the  most  but 
imitating  in  a  rough  way  the  work  of  central  nerve-cells.  We 
can  only  mention  a  few  points  to  illustrate  this. 

In  the  frog  a  succession  of  light  taps,  or  a  single  sharp  one 
("  Klopf versuch  "  of  Goltz),  will  usually  arrest  the  heart  reflexly ; 
though  sometimes  it  is  very  difficult  to  accomplish.  But  in  the 
fish  the  ease  with  which  the  heart  may  be  reflexly  inhibited  by 
gentle  stimulation  of  almost  any  portion  of  the  animal  is  won- 
derful. Again,  in  some  animals  the  vagus  arrests  the  heart  for 
only  a  brief  period,  when  it  breaks  away  into  its  usual  (but  in- 
creased) action. 

In  the  fish,  menobranchus,  and  probably  other  animals,  the 
irritability  of  some  subdivision  of  the  heart  is  lost  during  the 
vagus  inhibition — i.  e.,  it  does  not  x-espond  to  a  mechanical 
stimulus. 


256 


COMPARATIVE  PHYSIOLOGY. 


There  is  usually  a  certain  order  in  which  the  heart  recom- 
mences after  inhibition  (viz. ,  sinus,  auricles,  ventricles) ;  but 
there  are  variations  in  this,  also,  for  different  animals.  It  is 
also  a  fact  that  in  most  of  the  cold-blooded  animals  the  right 


S.  Vagus, 


Heart. 


Brain  above  Medulla. 


Cardie-inhibitory  Cen- 
ter in  Medulla  Ob- 


Afferent  Nerve. 


Outlying  Area  with  its 
Nerves. 


Fio.  310. —  Diagram  of  the  inhibitory  mechanism  of  the  heart.  The  arrows  indicate 
in  all  cases  the  path  the  nervous  impulses  take.  I.  Path  of  afferent  impulses 
from  I  he  heart  itself.  II.  Path  from  parts  of  the  brain  above  (or  anterior  to)  the 
vaso-motor  center.  A  similar  one  might,  of  course,  be  mapped  out  along  the 
spinal  cord.  III.  Path  from  some  peripheral  region.  The  downward  arrows  in- 
dicate the  course  of  efferent  impulses,  which  probably  usually  pass  by  both  vagi. 

vagus  is  more  efficient  than  the  left,  owing,  we  think,  not  to 
the  nerves  themselves  so  much  as  to  their  manner  of  distribu- 
tion in  the  heart—the  greater  portion  of  the  driving  part  of  the 


THE   CIRCULATION   OP  THE  BLOOD.  257' 

organ,so  to  speak,  being  supplied  by  tbe  right  nerve  ;  for,  when 
even  a  small  part  of  the  heart  is  arrested,  it  may  be  overcome  by 
the  action  of  a  larger  portion  of  the  same,  or  a  more  dominant 
region  (the  sinus  mostly). 

Conclusions. — The  inferences  from  tbe  facts  stated  in  the 
above  paragraphs  are  these  :  1.  There  is  in  the  medulla  a  col- 
lection of  cells  (center)  which  can  generate  impulses  that  reach 
the  heart  by  the  vagi  nerves  and  influence  its  muscular  tissue, 
though  whether  directly  or  through  the  intermediation  of 
nerve-cells  in  its  substance  is  uncertain.  It  may  possibly  be  in 
both  ways.  2.  This  center  (cardio-inhibitory)  may  be  influ- 
enced reflexly  by  influences  ascending  by  a  variety  of  nerves 
from  the  periphery,  including  paths  in  the  brain  itself,  as 
shown  by  the  influence  of  emotions  or  the  behavior  of  the 
heart.  3.  The  cardio-inhibitory  center  is  the  agent,  in  part, 
through  which  the  rhythm  of  the  heart  is  adapted  to  the  needs 
of  the  body.  4.  The  arrest,  on  direct  stimulation  of  the  heart, 
is  owing  to  the  effect  produced  on  the  terminal  fibers  of  the 
vagi,  as  shown  by  the  dilatation,  etc.,  corresponding  to  what 
takes  place  when  the  trunk  of  the  nerve  or  the  center  is  stimu- 
lated. 5.  The  quickening  of  the  heart,  following  section  of  the 
vagi,  seems  to  show  that  in  some  animals  the  inhibitory  center 
exercises  a  constant  regulative  influence  over  the  rhythm  of 
the  heart.  6.  The  irritability  and  dilatability  of  the  cardiac 
tissue  may  be  greatly  modified  dui'ing  vagus  inhibition.  Some- 
times this  is  evident  before  the  rhythm  itself  is  appreciably 
altered.  7.  The  heart-muscle  has  a  latent  period,  like  other 
kinds  of  muscle ;  and  cardiac  effects,  when  initiated,  last  a  vari- 
able period. 

There  are  many  other  obvious  conclusions,  which  the  stu- 
dent will  draw  for  himself. 

But  a  question  arises  in  regard  to  the  significance  of  the 
cardiac  arrest  under  these  circumstances,  and  the  altered  action 
that  follows.  The  fact  that,  when  the  heart  is  severed  from  the 
central  nervous  system  by  section  of  its  nerves,  profound 
changes  in  the  minute  structure  of  its  cells  ensue,  points  un- 
mistakably to  some  nutritive  influence  that  must  have  operated 
through  the  vagi  nerves.  That  stimulation  of  the  vagus  re- 
stores regularity  of  rhythm  and  strengthens  the  beat  of  the 
failing  heart,  is  also  very  suggestive.  That  many  disorders  of 
the  heart  are  coincident  with  periods  of  mental  anguish  or 
worry,  and  that  in  certain  cases  of  severe  mental  application 
17 


258  COMPARATIVE   PHYSIOLOGY. 

the  heart's  rhythm  lias  become  very  slow,  also  point  to  influ- 
ences of  a  central  origin  as  greatly  affecting  the  life-processes 
of  this  organ. 

It  has  been  shown  that  the  vagus  nerve  in  some  cold-blooded 
animals,  as  is  probable  also  in  the  higher  vertebrates,  consists 
of  two  sets  of  fibers — those  which  are  inhibitory  proper  and 
those  which  are  not,  but  belong  to  the  sympathetic  system. 

Sepai'ate  stimulation  of  the  former  favors  nutritive  processes, 
is  preservative  ;  of  the  latter,  destructive.  This  has  been  ex- 
pressed by  saying  that  the  former  favors  constrictive  (anabolic) 
metabolism  ;  the  latter  destructive  (katabolic)  metabolism.  It 
is  assumed  that  all  the  metabolism  of  the  body  may  be  repre- 
sented as  made  up  of  katabolic  following  anabolic  processes. 

Whether  such  a  view  of  metabolism  expresses  any  more 
than  a  sort  of  general  tendency  of  the  chemistry  of  the  body 
is  doubtful.  It  is  a  very  simple  representation  of  what  in  all 
probability  is  extremely  complex  ;  and  if  it  be  implied  that 
throughout  the  body  certain  steps  are  always  taken  upward  in 
construction  to  be  always  afterward  followed  by  certain  down- 
ward destructive  changes,  we  must  reject  it  as  too  rigid  and 
artificial  a  representation  of  natural  processes. 

We  think,  however,  that,  upon  all  the  evidence,  pathological 
and  clinical  as  well  as  physiological,  the  student  may  believe 
that  the  vagus  nerve,  like  the  other  nerves  of  the  body,  accord- 
ing to  our  own  theory,  exercises  a  constant  beneficial,  guiding 
— let  us  say  determining — influence  over  the  metabolism  of  the 
organ  it  supplies ;  and  we  here  suggest  that,  if  this  view  were 
applied  to  the  origin  and  course  of  cardiac  disease,  it  would 
result  in  a  gain  to  the  science  and  art  of  medicine. 


THE  ACCELERATOR  (AUGMENTOR)  NERVES  OF  THE 

HEART. 

It  has  been  known  for  many  years  that  in  the  dog,  cat, 
rabbit,  and  some  other  mammals,  there  are  nerves  proceeding 
from  certain  of  the  ganglia  of  the  sympathetic  chain  high  up, 
stimulation  of  which  lead  to  an  acceleration  of  the  heart-beat. 
Very  recently  these  nerves  have  been  traced  in  a  number  of 
cold-blooded  animals,  and  the  whole  subject  placed  on  a  broader 
and  sounder  basis. 

There  are  variations  in  the  distribution  of  these  nerves  for 
different  groups  of  animals,  but  it  will  suffice  if  we  indicate 


THE   CIRCULATION   OF   THE   BLOOD. 


259 


their  course  in  a  general  way,  without  special  reference  to  the 
variations  for  each  animal  group:  1.  These  nerves  emerge  from 
the  spinal   cord   (upper  dorsal  region),   and   proceed  upward 


Spinal  Cord. 


Accelerator  Center  in  Me- 
dulla. 


Superior  Cervical  Ganglion. 


Middle  Cervical  Ganglion. 


Inferior  Cervical  Ganglion. 


Basal  Ganglion    in  Region 
of  First  Rib. 


Accelerator  Nerves. 


Heart. 


Fig.  217.^Diagrani  to  illustrate  the  origin,  course,  etc.,  of  accelerator  impulses.  It 
will  be  understood  that  this  is  intended  to  indicate  the  general  plan,  and  not  pre- 
cisely what  takes  place  in  any  one  animal.  Thus,  while  the  accelerator  nerves 
may  arise  in  this  way,  it  is  not  meant  to  be  implied  that  the  heart  is  actually  sup- 
plied by  three  nerves  of  such  origin  in  any  case.  The  arrows,  as  before,  indicate 
the  path  of  the  impulses. 


before  being  distributed  to  the  heart.  2.  They  may  leave  for 
their  cardiac  destination  either  at  (a)  the  first  thoracic  (or  basal 
cardiac  ganglion,  as  it  might  be  named  in  this  case),  (6)  the  in- 
ferior cervical  ganglion,  (c)  the  annulus  of  Vieussens,  or  (d)  the 
middle  cervical  ganglion. 

It  follows  that  the  heart  may  be  made  to  do  increased  work 
in  three  ways  :     First,  the  relaxation  of  a  normal  inhibitory 


260 


COMPARATIVE   PHYSIOLOGY. 


control  through  the  vagus  nerve  by  the  cardio-inhibitory  cen- 
ter ;  second,  through  the  sympathetic  (motor)  fibers  in  the 
vagus  itself  :  and,  finally,  through  fibers  with  similar  action 
in  the  sympathetic  system,  usually  so  called. 

The  share  taken  by  these  factors  is  certainly  variable  in  dif- 
ferent species  of  animals,  and  it  is  likely  that  this  is  true  of  the 
same  animals  on  different  occasions.  It  is  also  conceivable, 
and  indeed  probable,  that  they  act  together  at  times,  the  inhibi- 
tory action  being  diminished  and  the  augmentor  influence  in- 
creased. 


THE   HEART   IN   RELATION   TO   BLOOD-PRESSURE. 

It  is  plain  that  all  the  other  conditions  throughout  the  cir- 
culatory system  remaining  the  same,  an  increase  in  either  the 
force  or  the  frequency  of  the  heart-beat  must  raise  the  blood- 
pressure.  But,  if  the  pressure  were  generally  raised  when  the 
heart  beats  rapidly,  it  would  fare  ill  with  the  aged,  the  elasticity 
of  their  arteries  being  usually  greatly  impaired.  As  a  matter  of 
fact  any  marked  rise  of  pressure  that  would  thus  occur  is  pre- 


>w~-s 


Fig.  218  —Tracing  from  a  rabbit,  showing  the  influence  of  cardiac  inhibition  on  blood- 
pressure.  The  fall  in  this  case  was  very  rapid,  owing  to  sudden  cessation  of  the 
heart-beat.  The  relative  emptiness  of  the  vessels  accounts  for  the  peculiar  char- 
acter of  the  curve  of  rising  blood-pressure  (Foster). 

vented  as  a  rule,  and  in  different  ways,  as  will  be  seen  ;  but,  so 
far  as  the  heart  is  concerned,  its  beat  is  usually  the  weaker  the 
more  rapid  it  is,  so  that  the  cardiac  rhythm  and  the  blood-press- 
ure are  in  inverse  proportion  to  each  other. 

By  what  method  is  the  heart's  action  tempered  to  the  condi- 


THE   CIRCULATION   OF  THE   BLOOD.  261 

tions  prevailing  at  the  time  in  the  other  parts  of  the  vascular 
system  ? 

The  matter  is  complex.  The  effect  of  vagus  stimulation  on 
the  blood-pressure  is  always  very  marked,  as  would  be  supposed. 

As  seen  in  the  tracing,  the  beats,  when  the  heart  commences 
its  action  again  tell  on  the  comparatively  slack  walls  of  the 
arteries,  distending  them  greatly,  and  this  may  be  made  evident 
by  the  sphymograph  as  well  as  the  manometer ;  indeed,  may  be 
evident  to  the  finger,  the  pulse  resembling  in  some  features  that 
following  excessive  loss  of  blood, 

If  the  heart  has  been  merely  slowed,  or  its  pulsation  weak- 
ened, the  effects  will  of  course  be  less  marked. 

The  Quantity  of  Blood.— The  blood-pressure  may  also  be 
augmented,  the  cardiac  frequency  remaining  the  same,  by  the 
quantity  of  blood  ejected  from  the  ventricles,  which  again 
depends  on  the  quantity  entering  them,  a  factor  determined 
by  the  condition  of  the  vessels,  and  to  this  we  shall  presently 
turn. 

In  consequence  of  changes  in  different  parts  of  the  system  by 
way  of  compensation,  results  follow  in  an  animal  which  might 
not  have  been  anticipated. 

Thus,  bleeding,  unless  to  a  dangerous  extreme,  does  not  lower 
the  blood-pressure  except  temporarily.  It  is  estimated  that  the 
body  can  adapt  itself  to  a  loss  of  as  much  as  3  per  cent  of  the 
body-weight. 

The  adaptation  is  probably  not  through  absorption  chiefly, 
but  through  constriction  of  the  vessels  by  the  vaso- motor 
nerves. 

Again,  an  injection  of  fluid  into  the  blood  does  not  cause  an 
appreciable  rise  of  blood-pressure,  so  long  as  the  nervous  svs- 
tem  is  intact  ;  but,  if  by  section  of  the  spinal  cord  the  vaso- 
motor influences  are  cut  off,  then  a  rise  may  take  place  to  the 
extent  of  2  to  3  per  cent  of  the  body -weight,  the  extra  quan- 
tity of  fluid  seeming  to  be  accommodated  in  the  capillaries  and 
smaller  veins.  These  facts  are  highly  significant  in  illustrat- 
ing the  adaptive  power  of  the  circulatory  system  (protective  in 
its  nature),  and  are  of  practical  importance  in  the  treatment  of 
disease. 

We  think  the  benefit  that  sometimes  follows  bleeding  has 
not  as  yet  received  an  adequate  explanation,  but  we  shall  not 
attempt  to  tackle  the  problem  now.  Changes  in  the  circulation 
depend  on  variations  in  the  size  of  the  blood-vessels. 


262  COMPARATIVE    PHYSIOLOGY. 

It  is  important  in  considering  this  subject  to  have  clear  no- 
tions of  the  structure  of  the  blood-vessels.  It  will  be  borne  in 
mind  that,  while  muscular  elements  are  perhaps  not  wholly 
lacking  in  any  of  the  arteries,  they  are  most  abundant  in  the 
smallest,  the  arterioles,  which  by  their  variations  in  size  are  best 
fitted  to  determine  the  quantity  of  blood  reaching  any  organ. 
It  is  well  known  that  nerves  derived  chiefly  from  the  sympa- 
thetic system  pass  to  blood-vessels,  though  their  exact  mode  of 
termination  is  obscure.  As  the  result  of  the  section  and  stimu- 
lation of  certain  nerves  the  following  inferences  have  been 
drawn  in  regard  to  the  nerves  supplying  blood-vessels. 

1.  There  are  vaso-motor  nerves  of  two  kinds — vaso-constrict- 
ors  and  vaso-dilators — which  may  exist  in  nerve-trunks  either 
separately  or  mingled.  Examples  of  the  former  are  found  in 
the  cervical  sympathetic,  splanchnic,  etc.,  of  the  latter  in  the 
chorda  tympani,  nerves  of  the  muscles  and  nervi  erigentes 
(from  the  first,  second,  and  third  sacral  nerves),  while  the  sci- 
atic seems  to  contain  both. 

2.  Impulses  are  constantly  passing  from  the  medullary  vaso- 
motor center  along  the  nerves  to  the  blood-vessels,  hence  their 
dilatation  after  section  of  the  nerves.  The  nerves  are  traceable 
to  the  spinal  cord,  and  in  some  part  of  their  course  run,  as  a 
rule,  in  the  sympathetic  system. 

3.  Impulses  pass  at  intervals  to  the  areas  of  distribution  of 
vaso-dilators  along  these  nerves,  the  effect  of  which  is  to  dilate 
the  vessels  through  their  influence,  as  in  other  cases,  on  the 
muscular  coat. 

It  is  inferred  that  there  are  vaso-motor  centers  in  the 
spinal  cord  which  are  usually  subordinated  to  the  main  center 
in  the  medulla,  but  which  in  the  absence  of  the  control  of  the 
chief  center  in  the  medulla  assume  an  independent  regulating 
influence. 

There  is  a  nerve  with  variable  origin,  course,  etc.,  in  differ- 
ent mammals,  but  in  the  rabbit  given  off  from  either  the  vagus, 
the  superior  laryngeal,  or  by  a  branch  from  each,  which,  run- 
ning near  the  sympathetic  nerve  and  the  carotid  artery,  reaches 
the  heart,  to  which  it  is  distributed.  This  is  known  as  the  de- 
pressor nerve. 

From  stimulation  of  the  central  end  of  this  nerve  results 
follow  which  warrant  the  conclusion  that  impulses  can  by  it 
reach  the  vaso-motor  center  in  the  medulla,  and  interfere  with 
(inhibit)  the  outflow  of  efferent,  constrictive,  or  tonic  impulses, 


THE   CIRCULATION   OP  THE   BLOOD. 


263 


which  start  from  the  vasomotor  center,  descend  the  cord,  and 
find  their  way  to  the  organs  of  a  definite  region,  in  consequence 
of  which  the  muscular  coats  of  the  arterioles  relax,  more  blood 
flows  to  this  area  which  is  very  large,  and  the  general  blood- 
pressure  is  lowered. 

Again,  if  the  central  end  of  one  of  the  main  nerves — e.  g., 
sciatic — be  stimulated,  a  marked  change  in  the  blood-pressure 


iso-motor  Center 
in  Medulla. 


Spinal  Cord 


Efferent  Vaso-mo 
tor  Nerve. 


Outlying  Vascularl 
Area.  ~v*-^!ti|ST 


Afferent  Nerve 
from  Periphery. 


Fig.  219.— Diagram  of  nervous  vasomotor  mechanism.    I.  Course  of  afferent  impulses 

from  the  heart  itself  along  the  depressor  nerve.     II.  Course  from  some  other  part 

of  the  brain.    III.  Course  from  some  peripheral  region  along  a  nerve  joining  the 

spinal  cord.    The  efferent  impulses  are  represented  as  passing  to  a  vascular  area, 

•  reduced  for  the  sake  of  simplicity  to  a  single  arteriole. 

results,  but  whether  in  the  direction  of  rise  or  fall  seems  to  de- 
pend upon  the  condition  of  the  central  nervous  system,  for,  with 
the  animal  under  the  influence  of  chloral,  there  is  a  fall;  if 
under  urari,  a  rise, 


264 


COMPARATIVE  PHYSIOLOGY. 


It  is  not  to  be  supposed  that  the  change  in  any  of  these 
cases  is  confined,  to  any  one  vascular  area  invariably,  but  that 
it  is  this  or  that,  according  to  the  nerve  stimulated,  the  condi- 


J 


fUUUUUUUUUVAJUUUUlAJlAAJLJLAJUUUULA^ 


Fig.  220.— Curve  of  blood-pressure  resulting  from  stimulation  of  the  central  end  of 
the  depressor  nerve.  To  be  read  from  right  to  left.  T  indicates  the  rate  at  which 
the  recording  surface  moved,  the  intervals  denoting  seconds.  At  V  the  current 
was  thrown  into  the  nerve,  and  shut  off  at  0.  The  result  appears  after  a  period 
of  latency,  and  outlasts  the  stimulus  (Poster) 

tion  of  the  centers,  and  a  number  of  other  circumstances. 
Moreover,  it  is  importaut  to  bear  in  mind  that  with  a  fall  of 
blood-pressure  in  one  region  there  may  be  a  corresponding  rise 
in  another.  With  these  considerations  in  mind,  it  will  be  ap- 
parent that  the  changes  in  the  vascular  system  during  the 
course  of  a  single  hour  are  of  the  most  complex  and  variable 
character. 

The  question   of  the  distribution  of  vaso-motor  nerves  to 
veins  is  one  to  which  a  definite  answer  can  not  be  given. 


THE   CAPILLARIES. 

The  cells  of  which  the  capillaries  are  composed  have  a  con- 
tractility of  their  own,  and  hence  the  caliber  of  the  capillaries 
is  not  determined  merely  by  the  arterial  pressure  or  any  similar 
mechanical  effect. 

Certain  abnormal  conditions,  induced  in  these  vessels  by 
the  application  of  irritants,  cause  changes  in  the  blood-flow, 
which  can  not  be  explained  apart  from  the  vitality  of  the  ves- 
sels themselves. 

Watched  through  the  microscope  under  such  circumstances, 


THE  CIRCULATION   OP  THE  BLOOD.  265 

the  blood-corpuscles  no  longer  pursue  their  usual  course  in  the 
mid-stream,  but  seem  to  be  generally  distributed  and  to  hug  the 
walls,  one  result  of  which  is  a  slowing  of  the  stream,  wholly 
independent  of  events  taking  place  in  other  vessels.  It  is  thus 
seen  that  in  this  condition  (stasis)  the  capillaries  have  an  in- 
dependent influence  essentially  vital.  We  say  independent,  for 
it  is  still  an  open  question  whether  nerves  are  distributed  to 
capillaries  or  not.  That  inflammation,  in  which  also  the  walls 
undergo  such  serious  changes  that  white  and  even  red  blood- 
cells  may  pass  through  them  {diapedesis),  is  not  uninfluenced 
by  the  nervous  system,  possibly  induced  through  it  in  certain 
cases,  if  not  all,  seems  more  than  probable. 

But  when  we  consider  the  lymphatic  system  new  light  will, 
it  is  hoped,  be  thrown  upon  the  subject  of  the  nature  and  the 
influences  which  modify  the  capillaries.  One  thing  will  be 
clear  from  what  has  been  said,  that  even  normally  the  capil- 
laries must  exert  an  influence  of  the  nature  of  a  resistance, 
owing  to  their  peculiar  vital  properties ;  and,  as  we  have  already 
intimated  such  considerations  should  not  be  excluded  from  any 
conclusions  we  may  draw  in  regard  to  tubes  that  are  made  up 
of  living  cells,  whether  arteries,  veins,  or  capillaries,  though 
manifestly  the  applicability  to  capillaries,  with  their  less  modi- 
fied or  more  primitive  structure,  is  stronger. 

It  has  now  become  clear  that  the  circulation  may  he  modi- 
fied either  centrally  or  peripherally;  that  a  change  is  never 
purely  local,  but  is  correlated  with  other  changes  ;  that  the 
whole  is,  in  the  higher  animals,  directly  under  the  dominion  of 
the  central  nervous  system  ;  and  that  it  is  through  this  part 
chiefly  that  harmony  in  the  vascular  as  in  other  systems  and 
with  other  systems  is  established.  To  have  adequately  grasped 
this  conception  is  worth  more  than  a  knowledge  of  countless 
details. 

SPECIAL   CONSIDERATIONS. 

Pathological —Changes  may  take  place  either  in  the  sub- 
stance of  the  cardiac  muscle,  in  the  valves,  or  in  the  blood-ves- 
sels, of  a  nature  unfavorable  to  the  welfare  of  the  body.  Some 
of  these  have  been  incidentally  referred  to  already. 

Hypertrophy,  or  an  increase  in  the  tissue  of  the  heart,  is 
generally  dependent  on  increased  resistance,  either  within  or 
without  the  heart,  in  the  region  of  the  arterioles  or  capillaries. 
Imperfections  of  the  aortic  valves  may  permit  of  regurgitation 


266  COMPARATIVE   PHYSIOLOGY. 

of  blood,  entailing  an  extra  effort  if  it  is  to  be  expelled  in  addi- 
tion to  the  usual  quantity,  which  again  leads  to  hypertrophy ; 
but  this  is  often  suceeded  by  dilatation  of  the  chambers  of  the 
heart  one  after  the  other,  and  a  host  of  evils  growing  out  of 
this,  largely  dependent  on  imperfect  venous  circulation,  and 
increased  venous  pressure.  And  it  may  be  here  noticed  that 
arterial  and  venous  pressures  are,  as  a  general  rule,  in  inverse 
proportion  to  each  other. 

If  the  quantity  of  blood  in  the  ventricle,  in  consequence  of 
regurgitation,  should  prove  to  be  greater  than  it  can  lift  (eject), 
the  heart  ceases  to  beat  in  diastole ;  hence  some  of  the  sudden 
deaths  from  disease  of  the  aortic  valves. 

As  a  result  of  fatty,  or  other  forms  of  degeneration,  the 
heart  may  suddenly  rupture  under  strains. 

Actual  experiment  on  the  arteries  of  animals  recently  dead, 
including  men,  shows  that  the  elasticity  of  the  arteries  of  even 
adult  mammals  is  as  perfect  as  that  of  the  vessels  of  the  child, 
so  that  man  ranks  lower  than  other  animals  in  this  respect. 

After  a  certain  period  of  life  the  loss  of  arterial  elasticity  is 
considerable  and  progressive.  The  arteries  may  undergo  a  de- 
generation from  fatty  changes  or  deposit  of  lime  ;  such  vessels 
are,  of  course,  liable  to  rupture  ;  hence  one  of  the  modes  of 
death  among  old  animals  is  from  paralysis  traceable  to  rupture 
of  vessels  in  the  brain. 

These  and  other  changes  also  cause  the  heart  more  work, 
and  may  lead  to  hypertrophy.  Even  in  young  animals  the 
strain  of  a  prolonged  racing  career  may  entail  hypertrophy  or 
some  other  form  of  heart-disease. 

We  mention  such  facts  as  these  to  show  the  more  clearly 
how  important  is  balance  and  the  power  of  ready  adaptation 
in  all  parts  of  the  circulation  to  the  maintenance  of  a  healthy 
condition  of  body. 

The  heart  is  itself  nourished  through  the  coronary  arteries  ; 
so  that  morbid  alterations  in  these  vessels  cause,  if  not  sudden 
and  painful  death,  at  least  nutritive  changes  in  the  heart-sub- 
stance, which  may  lead  to  a  dramatic  end  or  to  a  slow  impair- 
ment of  cardiac  power,  etc. 

Personal  Observation. — The  circulation  is  one  of  those  de- 
partments of  physiology  in  which  the  student  may  verify  much 
upon  his  own  person.  The  cardiac  impulse,  the  heart's  sounds 
(with  a  double  stethoscope),  the  pulse— its  nature  and  changes 
with  circumstances,  the  venous  circulation,  and  many   other 


THE   CIRCULATION    OF   THE   BLOOD, 


267 


subjects,  are  all  easy  of  observation,  and  after  a  little  practice 
without  liability  of  causing  those  aberrations  due  to  the  atten- 
tion being  drawn  to  one's  self. 

The  observations  need  not,  of  course,  be  confined  to  the  stu- 
dent's own  person  ;  it  is,  however,  very  important  that  the  nor- 
mal should  be  known  before  the  observer  is  introduced  to  cases 
of  disease.  Frequent  comparison  of  the  natural  and  the  dis- 
eased condition  renders  physiology,  pathology,  and  clinical 
medicine  much  good  service.  We  again  urge  upon  the  student 
to  try  to  form  increasingly  vivid  and  correct  mental  pictures  of 
the  circulation  under  its  many  changes. 

Comparative,— An  interesting  arrangement  of  blood-vessels, 
known  as  a  rete  mirabile,  occurs  in  every  main  group  of  verte- 


Fig  221  —Rete  mirabile  of  sheep,  seen  in  profile  (after  Chauveau).  The  larger  rete  is 
in  connection  with  the  encephalic  arteries;  the  smaller,  the  ophthalmic.  The  large 
artery  is  the  carotid. 

brates.  An  artery  breaks  up  into  a  great  number  of  vessels  of 
nearly  the  same  size,  which  terminate,  abruptly  and  without 
capillaries,  in  another  arterial  trunk. 

They  are  found  in  a  variety  of  situations,  as  on  the  carotid 
and  vertebrate  arteries  of  animals  that  naturally  feed  from  the 
ground  for  long  periods  together,  as  the  ruminants  ;  in  the 
sloth,  that  hangs  from  trees  ;  in  the  legs  of  swans,  geese,  etc. ; 
in  the  horse's  foot,  in  which  the  arteries  break  up  into  many 
small  divisions.     It  has  been  suggested  that  these  arrangements 


268  COMPARATIVE   PHYSIOLOGY. 

permit  of  a  supply  of  arterial  blood  being  maintained  without 
congestion  of  the  parts.  Very  marked  tortuosity  of  vessels,  as 
in  the  seal,  the  carotid  of  which  is  said  to  be  forty  times  as  long 


Fig.  222. — Section  of  a  lymphatic  rete  mirabile,  from  the  popliteal  space  (after  Chan- 
veau).  a,  a,  afferent  vessels,  b,  b,  efferent  vessels.  The  whole  very  strongly  sug- 
gests a  crude  form  of  lymphatic  gland. 

as  the  space  it  traverses,  in  all  probability  serves  the  same  pur- 
pose. 

Evolution. — The  comparative  sketch  we  have  given  of  the 
vascular  system  will  doubtless  suggest  a  gradual  evolution.  We 
observe  throughout  a  dependence  and  resemblance  which  we 
think  can  not  be  otherwise  explained.  The  similarity  of  the 
fcetal  circulation  in  the  mammal  to  the  permanent  circula- 
tion of  lower  groups  has  much  meaning.  Even  in  the  highest 
form  of  heart  the  original  pulsatile  tube  is  not  lost.  The  great 
veins  still  contract  in  the  mammal  ;  the  sinus  venosus  is  proba- 
bly the  result  of  blending  and  expansion.  The  later  differentia- 
tions of  the  parts  of  the  heart  are  clearly  related  to  the  adapta- 
tion to  altered  surroundings.  Such  is  seen  in  the  fcetal  heart 
and  circulation,  and  has  probably  been  the  determining  cause 
of  the  forms  which  the  circulatory  organs  have  assumed. 

It  is  a  fact  that  the  part  of  the  heart  that  survives  the  long- 
est under  adverse  conditions  is  that  which  bears  the  stamp  of 
greatest  ancestral  antiquity.     It  (the  sinus  venosus)  may  not 


THE   CIRCULATION   OF   THE   BLOOD. 


269 


be  less  under  nervous  control,  but  it  certainly  is  least  dependent 
on  tbe  nervous  system,  and  bas  tbe  greatest  automaticity. 

The  law  of  rhythm  in  organic  nature  finds  some  of  its  most 
evident  exemplifications  in  tbe  circulation.  Most  of  tbe 
rhytbms  are  com- 
pound, one  being  |illifl|HflJf 
blended  with  or  su- 
perimposed on  an- 
other. Even  tbe  ap- 
parent irregularities 
of  tbe  normal  heart 
are  rhythmical,  such 
as  the  very  marked 
slowing  and  other 
changes  accompany- 
ing expiration,  espe- 
cially in  some  ani- 
mals. 

We  trust  we  have 
made  it  evident  that 
the  greatest  allow- 
ance must  be  made 
for  the  animal  group, 
and  some  even  for 
the  individual,  in  es- 
timating any  one  of 
the  factors  of  the  cir- 
culation. We  know 
a  good  deal  at  present  of  cardiac  physiology,  but  we  do  not 
know  a  physiology  of  "  the  heart "  in  the  sense  in  which  we 
understand  that  term  to  have  been  used  till  recently — i.  e.,  we 
are  not  in  a  position  to  state  tbe  laws  that  apply  to  all  forms  of 
heart. 

Summary  of  the  Physiology  of  the  Circulation.— In  the 
mammal  the  circulatory  apparatus  forms  a  closed  system  con- 
sisting of  a  central  pump  or  heart,  arteries,  capillaries,  and 
veins.  All  the  parts  of  the  vascular  system  are  elastic,  but  this 
property  is  most  developed  in  the  arteries. 

Since  the  tissue-lymph  is  prepared  from  the  blood  in  the 
capillaries,  it  may  be  said  that  the  whole  circulatory  system 
exists  for  these  vessels. 

As  a  result  of  the  action  of  an  intermittent  pump  on  elastic 


Fig.  283.— Veins  of  the  foot  of  the  horse  (after  Chau- 
veau). 


270  COMPARATIVE  PHYSIOLOGY. 

vessels  against  peripheral  resistance,  in  consequence  of  which 
the  arteries  are  always  kept  more  than  full  (distended),  the 
flow  through  the  capillaries  and  veins  is  constant — a  very  great 
advantage,  enabling  the  capillaries  to  accomplish  their  work  of 
feeding  the  ever-hungry  tissues.  While  physical  forces  play  a 
very  prominent  part  in  the  circulation  of  the  blood,  vital  ones 
must  not  be  ignored.  They  lie  at  the  foundation  of  the  whole, 
here  as  elsewhere,  and  must  be  taken  into  the  account  in  every 
explanation. 

As  a  consequence  of  the  anatomical,  physical,  and  vital  char- 
acters of  the  circulatory  system,  it  follows  that  the  velocity  of 
the  blood  is  greatest  in  the  arteries,  least  in  the  capillaries,  and 
intermediate  in  the  veins. 

The  veins  with  their  valves,  their  superficial  position  and 
thinner  walls,  make  up  a  set  of  conditions  favoring  the  onflow 
of  the  blood,  especially  under  muscular  exercise. 

In  the  mammal  the  circulatory  system,  by  reason  of  its  con- 
nections with  the  digestive,  respiratory,  and  lymphatic  systems, 
and  in  a  lesser  degree  with  all  parts  of  the  body,  especially  the 
glandular  organs,  maintains  at  once  the  usefulness  and  the  fit- 
ness of  the  blood. 

The  arterioles,  by  virtue  of  their  highly  developed  muscular 
coat,  are  enabled  to  regulate  the  blood-supply  to  every  part,  in 
obedience  to  the  nervous  system. 

The  blood  exercises  a  certain  pressure  on  the  walls  of  all 
parts  of  the  vascular  system,  which  is  greatest  in  the  heart  it- 
self, high  in  the  arteries,  lower  in  the  capillaries,  and  lowest  in 
the  veins,  in  the  largest  of  which  it  may  be  less  than  the  atmos- 
pheric pressure,  or  negative.  The  heart  in  the  mammal  consists 
of  four  perfectly  separated  chambers,  each  upper  and  each 
lower  pair  working  synchronously,  intermixture  of  arterial 
and  venous  blood  being  prevented  by  septa  and  interference  in 
working  by  valves.  The  heart  is  a  force-pump  chiefly,  but,  to 
some  extent,  a  suction-pump  also,  though  its  power  as  such 
purely  from  its  own  action  and  independent  of  the  respiratory 
movements  of  the  chest  is  slight  under  ordinary  circumstances. 
In  consequence  of  the  lesser  resistance  in  the  pulmonary  divis- 
ion of  the  circulation,  the  blood-pressure  within  the  heart  is 
much  less  in  the  right  than  in  the  left  ventricle — a  fact  in  har- 
mony with  and  causative  of  the  greater  thickness  of  the  walls 
of  the  latter;  for  in  the  foetus,  in  which  the  conditions  are  dif- 
ferent, this  distinction  does  not  hold. 


THE  CIRCULATION   OF   THE   BLOOD.  271 

The  ventricles  usually  completely  empty  themselves  of 
blood  and  maintain  their  systolic  contraction  even  after  this 
has  been  effected.  The  contraction  of  the  heart,  which  really 
begins  in  the  great  veins  near  their  junction  with  the  auri- 
cles (that  do  not  fully  empty  themselves),  is  at  once  fol- 
lowed up  by  the  auricular  and  ventricular  contraction,  the 
whole  constituting  one  long  peristaltic  wave.  Then  follows 
the  cardiac  pause,  which  is  of  longer  duration  than  the  entire 
systole. 

When  the  heart  contracts  it  hardens,  owing  to  closing  on  a 
non-compressible  fluid  dammed  back  within  its  walls  by  resist- 
ance a  f route.  At  the  same  time  the  hand  placed  on  the  chest- 
walls  over  the  heart  is  sensible  of  the  cardiac  impulse,  owing 
to  what  has  just  been  mentioned.  The  systole  of  the  chambers 
of  the  heart  gives  rise  to  a  first  and  a  second  sound,  so  called, 
caused  by  several  events  combined,  in  which,  however,  the  ten- 
sion of  the  valves  must  take  a  prominent  share.  The  work  of 
the  heart  is  dependent  on  the  quantity  of  blood  it  ejects  and 
the  pressure  against  which  it  acts.  The  pulse  is  an  elevation 
of  the  arterial  wall,  occurring  with  each  heart-beat,  in  conse- 
quence of  the  passage  of  a  wave  over  the  general  blood- stream. 
There  is  a  distention  of  the  entire  arterial  system  in  every  direc- 
tion. The  pulse  travels  with  extreme  velocity  as  compared  with 
the  blood-current.  The  heart-beat  varies  in  force,  frequency, 
duration,  etc.,  and  with  age,  sex,  posture,  and  numerous  other 
circumstances. 

The  whole  of  the  circulatory  system  is  regulated  by  the  cen- 
tral nervous  system  through  nerves.  There  is  in  the  medulla 
oblongata  a  small  collection  of  nerve-cells  making  up  the 
cardio-inhibitory  center.  This  center,  with  varying  degrees  of 
constancy,  depending  on  the  group  of  animals  and  the  needs 
of  the  organism,  sends  forth  impulses  (which  modify  the  beat 
of  the  heart  in  force  and  frequency)  through  the  vagi  nerves. 
There  are  nerves  of  the  sympathetic  system  with  a  center  in 
the  cervical  spinal  cord,  and  possibly  another  in  the  medulla, 
which  are  capable  of  originating  either  an  acceleration  of  the 
heart  rhythm  or  an  increase  of  the  force  of  the  beat,  or  both 
together,  known  as  accelerators  or  augmentors.  In  the  verte- 
brates thus  far  examined  the  vagus  is  in  reality  a  vago-sympa- 
thetic  nerve,  containing  inhibitory  fibers  proper,  and  sympa- 
thetic, accelerator,  or  motor  fibers. 

The  inhibitory  fibers  can  arrest,  slow,  or  weaken  the  cardiac 


272  COMPARATIVE   PHYSIOLOGY. 

beat ;  the  sympathetic  accelerate  it  or  augment  its  force.    When 
both  are  stimulated  together,  the  inhibitory  prevail. 

These  nerves,  as  also  the  accelerators,  exercise  a  profound 
influence  upon  the  nutrition  of  the  heart,  and  affect  its  electri- 
cal condition  when  stimulated,  and  we  may  believe  when  influ- 
enced by  their  own  centers. 

The  inhibitory  fibers  tend  to  preserve  and  restore  cardiac 
energy ;  the  sympathetic,  whether  in  the  vagus  or  as  the  aug- 
mentors,  the  reverse.  The  vagus  nerve  (and  probably  the  de- 
pressor) acts  as  an  afferent,  cardiac  sensory  nerve  reporting  on 
the  intra-cardiac  pressure,  etc.,  and  so  enabling  the  vaso- 
motor and  cardio-inhibitory  centers,  which  are,  it  would  seem, 
capable  of  related  and  harmonious  action  to  act  for  the  general 
good. 

The  arterioles  must  be  conceived  as  undergoing  very  fre- 
quent changes  of  caliber.  They  are  governed  by  the  vaso-motor 
center,  situated  in  the  medulla,  and  possibly  certain  subordi- 
nate centers  in  the  spinal  cord,  through  vaso-motor  nerves. 
These  are  (a)  vaso-constrictors,  which  maintain  a  constant  but 
variable  degree  of  contraction  of  the  muscle-cells  of  the  vessels ; 
(b)  vaso-dilators,  which  are  not  in  constant  functional  activity ; 
and  (c)  mixed  nerves,  with  both  kinds.  An  inherited  tendency 
to  rhythmical  contraction  throughout  the  entire  vascular  sys- 
tem, including  the  vessels,  must  be  taken  into  account. 

The  depressor  nerve  acts  by  lessening  the  tonic  contraction 
of  (dilating)  the  vessels  of  the  splanchnic  area  especially. 

It  is  important  to  remember  that  all  the  changes  of  the 
vascular  system,  so  long  as  the  nervous  system  is  intact — i.  e., 
so  long  as  an  animal  is  normal— are  correlated  ;  and  that  the 
action  of  such  nerves  as  the  depressor  is  to  be  taken  rather  as 
an  example  of  how  some  of  these  changes  are  brought  about, 
mere  chapters  in  an  incomplete  but  voluminous  history,  if  we 
could  but  write  it  all.  The  changes  in  blood-pressure,  by  the 
addition  or  removal  of  a  considerable  quantity  of  blood,  are 
slight,  owing  to  the  sort  of  adaptation  referred  to  above,  effected 
through  the  nervous  system.  Finally,  the  capillary  circulation, 
when  studied  microscopically,  and  especially  in  disordered  con- 
ditions, shows  clearly  that  the  vital  properties  of  these  vessels 
have  an  important  share  in  determining  the  character  of  the 
circulation  in  themselves  directly  and  elsewhere  indirectly. 

The  study  of  the  circulation ,  in  other  groups  shows  that 
below  birds  the  arterial  and  venous  blood  undergoes  mixture 


THE  CIRCULATION  OF   THE   BLOOD.  273 

somewhere,  usually  in  the  heart,  but  that  in  all  the  vertebrates 
the  best  blood  is  invariably  that  which  passes  to  the  head  and 
upper  regions  of  the  body.  The  deficiencies  in  the  heart,  owing 
to  the  imperfections  of  valves,  septa,  etc.,  are  in  part  counter- 
acted in  some  groups  by  pressure  relations,  the  blood  always 
flowing  in  the  direction  of  least  resistance,  so  that  the  above- 
mentioned  result  is  achieved. 

Capillaries  are  wanting  in  most  of  the  invertebrates,  the 
•  blood  flowing  from  the  arteries  into  spaces  (sinuses)  in  the  tis- 
sues. It  is  to  be  noted  that  a  modified  blood  (lymph)  is  also 
found  in  the  interspaces  of  the  cells  of  organs.  Indeed,  the 
circulatory  system  of  lower  forms  is  in  many  respects  analogous 
to  the  lymphatic  system  of  higher  ones. 


18 


DIGESTION   OF   FOOD. 


The  processes  of  digestion  may  be  considered  as  having  for 
their  end  the  preparation  of  food  for  entrance  into  the  blood. 

This  is  in  part  attained  when  the  insoluble  parts  have  been 
rendered  soluble.  At  this  stage  it  becomes  necessary  to  inquire 
as  to  what  constitutes  food  or  a  food. 

Inasmuch  as  animals,  unlike  plants,  derive  none  of  their 
food  from  the  atmosphere,  it  is  manifest  that  what  they  take  in 
by  the  mouth  must  contain  every  chemical  element,  in  some 
form,  that  enters  into  the  composition  of  the  body. 

But  actual  experience  demonstrates  that  the  food  of  animals 
must,  if  we  except  certain  salts  and  water,  be  in  organized 
form — i.  e.,  it  must  approximate  to  the  condition  of  the  tissues 
of  the  body  in  a  large  degree.  Plants,  in  fact,  are  necessary  to 
animals  in  working  up  the  elements  of  the  earth  and  air  into 
form  suitable  for  them. 

Foodstuffs  are  divisible  into  : 

I.  Organic. 

1.  Nitrogenous,   (a)  Albumins;  (b)  Albuminoids  (as gelatin). 

2.  Non-nitrogenous,     (a)  Carbohydrates  (sugars,  starches) ; 

(b)  Fats. 

II.  Inorganic. 

1.  Water. 

2.  Salts. 

Animals  may  derive  the  whole  of  their  food  from  the  bodies 
of  other  animals  (carnivora) ;  from  vegetable  matter  exclusively 
(herbivora) ;  or  from  a  mixture  of  the  animal  and  vegetable,  as 
in  the  case  of  the  pig,  bear,  and  man  himself  (omnivora). 

It  has  been  found  by  feeding  experiments,  carried  out  mostly 
on  dogs,  that  animals  die  when  they  lack  any  one  of  the  con- 
stituents of  food,  though  they  live  longer  on  the  nitrogenous 
than  any  other  kind.  In  some  instances,  as  when  fed  on  gela- 
tin and  water,  or  sugar  and  water,  the  animals  died  almost  as 


DIGESTION   OP   FOOD. 


275 


soon  as  if  they  had  been  wholly  deprived  of  food.  But  it  has 
also  been  observed  that  some  animals  will  all  but  starve  rather 
than  eat  certain  kinds  of  food,  though  chemically  sufficient. 
We  must  thus  recognize  something  more  in  an  animal  than 
merely  the  mechanical  and  chemical  processes  which  suffice  to 
accomplish  digestion  in  the  laboratory.  A  food  must  be  not 
only  sufficient  from  the  chemical  and  physical  point  of  view, 
but  be  capable  of  being  acted  on  by  the  digestive  juices,  and  of 
such  a  nature  as  to  suit  the  particular  animal  that  eats  it. 

To  illustrate,  bones  may  be  masticated  and  readily  digested 
by  a  hyena,  but  not  by  an  ox  or  by  man,  though  they  meet  the 
conditions  of  a  food  in  containing  all  the  requisite  constituents. 
Further,  the  food  that  one  man  digests  readily  is  scarcely  di- 
gestible at  all  by  another  ;  and  it  is  within  the  experience  of 
every  one  that  a  frequent  change  of  diet  is  absolutely  necessary. 

Since  all  mammals,  for  a  considerable  period  of  their  exist- 
ence, feed  upon  milk  exclusively,  this  must  represent  a  perfect 
or  typical  food.  It  will  be  worth  while  to  examine  the  compo- 
sition of  milk.  The  various  substances  composing  it,  and  their 
relative  proportions  for  different  animals,  may  be  seen  from  the 
following  table,  which  is  based  on  a  total  of  1,000  parts  : 


CONSTITUENTS. 

Human. 

Cow.                   Goat. 

Ass. 

Water 

889-08 

857-05              863-58 

910-24 

Casein 

j-      39-24 

26-66 

43-64 

1-38 

j      48-28               33-60 

}         5-76                12-99 

4305               43-57 

40-37                40-04 

5-48                  622 

j-     20-18 
12-56 

Albumin 

Butter 

Milk-sugar 

Salts 

j-     57-02 

Total  solids 

110-92 

142  95              136-42 

89  76 

The  following  table,  giving  the  percentage  composition  of 
the  milk  of  different  animals,  may  prove  instructive. 


Woman. 

Cow. 

Mare. 

Bitch. 

Pats 

2-00 
2-75 
0-25 
5-00 

4-00 
4-00 
0-60 
4-40 

2-50 
2-00 
0-50 
5-00 

10-00 
10-00 

Salts 

Sugar  

0-50 
3-50 

Total  solids 

10-00 
90-00 

13-00 

87-00 

10-00 
90-00 

24-00 

Water 

76-00 

276  COMPARATIVE   PHYSIOLOGY. 

1.  The  proteids  of  milk  are  : 

(a.)  An  albumin  very  like  serum-albumin. 

(p.)  Casein,  normally  in  suspension,  in  the  form  of  extremely 
minute  particles,  "which  contributes  to  the  opacity  of  milk. 

It  can  be  removed  by  filtration  through  porcelain  ;  and  pre- 
cipitated or  coagulated  by  acids  and  by  rennet,  an  extract  of 
the  mucus  membrane  of  the  calf's  stomach.  After  this  coagu- 
lation, whey,  a  fluid  more  or  less  clear,  separates,  which  con- 
tains the  salts  and  sugar  of  milk  and  most  of  the  water.  Much 
of  the  fat  is  entangled  with  the  casein. 

Casein,  with  some  fat,  makes  up  the  greater  part  of  cheese. 

2.  Fats. — Milk  is  an  emulsion — i.  e.,  contains  fat  suspended 
in  a  fine  state  of  division.  The  globules,  which  vary  greatly  in 
size,  are  surrounded  by  an  envelope  of  proteid  matter.  This 
covering  is  broken  up  by  churning,  allowing  the  fatty  globules 
to  run  together  and  form  butter. 

Butter  consists  chiefly  of  olein,  palmitin,  and  stearin,  but 
contains  in  smaller  quantity  a  variety  of  other  fats.  The  ran- 
cidity of  butter  is  due  to  the  presence  of  free  fatty  acids,  espe- 
cially butyric. 

The  fat  of  milk  usually  rises  to  the  surface  as  cream  when 
milk  is  allowed  to  stand. 

3.  Milk-sugar,  which  is  converted  into  lactic  acid,  probably 
by  the  agency  of  some  form  of  micro-organism,  thus  furnish- 
ing acid  sufficient  to  cause  the  precipitation  or  coagulation  of 
the  casein. 

Milk,  when  fresh,  should  be  neutral  or  faintly  alkaline. 

4.  Salts  (and  other  extractives),  consisting  of  phosphates  of 
calcium,  potassium,  and  magnesium,  potassium  chloride,  with 
traces  of  iron  and  other  substances. 

It  can  be  readily  understood  why  animals  fed  on  milk 
rarely  suffer  from  that  deficiency  of  calcium  salts  in  the  bones 
leading  to  rickets,  so  common  in  the  ill-fed.  It  thus  appears 
that  milk  contains  all  the  constituents  requisite  for  the  building 
up  of  the  healthy  mammalian  body;  and  experiments  prove 
that  these  exist  in  proper  proportions  and  in  a  readily  digestible 
form.  The  author  has  found  that  a  lai'ge  number  of  animals, 
into  the  usual  food  of  which,  in  the  adult  form,  milk  does  not 
enter,  like  most  of  our  wild  mammals,  as  well  as  most  birds, 
will  not  only  take  milk  but  soon  learn  to  like  it,  and  thrive  well 
upon  it.  Since  the  embryo  chick  lives  upon  the  egg,  it  might 
have  been  supposed  that  eggs  would  form  excellent  food  for 


DIGESTION  OF  FOOD. 


277 


adult  animals,  and  common  experience  proves  this  to  be  the 
case ;  while  chemical  analysis  shows  that  they,  like  milk,  con- 
tain all  the  necessary  food  constituents.  Meat  (muscle,  with 
fat  chiefly)    is  also,  of  course,  a  valuable  food,  abounding  in 

Animal  Foods. 

Explanation  of  the  signs. 


Proteids.    Albuminoids.    N-free  org.  bodies.        Salts. 

I2 M^jwsraiiiiii 


55 


73.5 


1' 

I" 


■».. 


0.4 


Vegetable  Foods. 

Explanation  of  the  signs. 


Tl'heaten-bread. 

Peas. 

Rice. 

Potatoes. 

Wliite  Turnip. 

Cauliflower. 

Beer. 


41.3 


13 


Digestible       Non-digestible 
N-free  organ  bodies. 


90.5 


90 


Fig.  234  (Landois). 


Salts. 

Illllllllllllllllllll 


1» 

2.5 


i-lllllllill 


0.5: 


I 


0.2 


10.5. 
I1 


I0,5 


proteids.    Cereals  contain  starch  in  large  proportion,  but  also  a 
mixture  of  proteids.     Green  vegetables  contain  little  actual  nu- 


278 


COMPARATIVE   PHYSIOLOGY. 


tritive  material,  but  are  useful  in  furnishing  salts  and  special 
substances,  as  certain  compounds  of  sulphur  which,  in  some  ill- 
understood  way,  act  beneficially  on  the  metabolism  of  the  body. 
They  also  seem  to  stimulate  the  flow  of  healthy  digestive  fluids. 
Condiments  act  chiefly,  perhaps,  in  the  latter  way.  Tea,  coffee, 
etc.,  contain  alkaloids,  which  it  is  likely  have  a  conservative 
effect  on  tissue  waste,  but  we  really  know  very  little  as  to  how 
it  is  that  they  prove  so  beneficial.  Though  they  are  recognized 
to  have  a  powerful  effect  on  the  nervous  system  as  stimulants, 
nevertheless  it  would  be  erroneous  to  suppose  that  their  action 
was  confined  to  this  alone. 

The  accompanying  diagrams  will  serve  to  represent  to  the 
eye  the  relative  proportions  of  the  food-essentials  in  various 
articles  of  diet. 


Pig.  225. — Alimentary  canal  of  embryo  while  the  rudimentary  mid-gut  is  still  in  con- 
tinuity with  yelk-sac  (KQlliker,  after  Bischoff).  A.  View  from  before,  a,  pharyn- 
geal plates;  b,  pharynx;  c,  c,  diverticula  forming  the  lungs;  d,  stomach;  /,  diver- 
ticula of  liver;  g,  membrane  torn  from  yelk-sac;  A,  hind  gut.  B.  Longitudinal 
section,    a,  diverticulum  of  a  lung;  b,  stomach;  c,  liver;  d,  yelk-sac. 


It  is  plain  that  if,  in  the  digestive  tract,  foods  are  changed 
in  solubility  and  actual  chemical  constitution,  this  must  have 
been  brought  about  by  chemical  agencies.  That  food  is  broken 
up  at  the  very  commencement  of  the  alimentary  tract  is  a 
matter  of  common  observation;  and  that  there  should  be  a 
gradual  movement  of  the  food  from  one  part  of  the  canal  to 
another,  where  a  different  fluid  is  secreted,  would  be  expected. 
As  a  matter  of  fact,  mechanical  and  chemical  forces  play  a 


DIGESTION   OF  FOOD. 


279 


large  part  in  the  actual  preparation  of  the  food  for  absorption. 
Behind  these  lie,  of  course,  the  vital  properties  of  the  glands, 
which  prepare  the  active  fluids  from 
the  blood,  so  that  a  study  of  diges- 
tion naturally  divides  itself  into  the 
consideration  of — 1.  The  digestive 
juices;  2.  The  secretory  processes; 
and,  3.  The  muscular  and  nervous 
mechanism  by  which  the  food  is 
carried  from  one  part  of  the  digest- 
ive tract  to  another,  and  the  waste 
matter  finally  expelled. 

Embryological—  The  alimentary 
tract,  as  we  have  seen,  is  formed  by 
an  infolding  of  the  splanchnopleure, 
and,  according  as  the  growth  is 
more  or  less  marked,  does  the  canal 
become  tortuous  or  remain  some- 
what straight.  The  alimentary  tract 
of  a  mammal  passes  through  stages 
of  development  which  correspond  with  the  permanent  form  of 
other  groups  of  vertebrates,  according  to  a  general  law  of  evo- 
lution.    Inasmuch  as  the  embryonic  gut  is  formed  of  mesoblast 


Fig.  226. — Diagram  of  alimentary 
canal  of  chick  at  fourth  day 
(Foster  and  Balfour,  after  GOt- 
te).  Ig,  diverticulum  of  one 
lung;  St,  stomach;  I,  liver;  p, 
pancreas. 


Fie.  227.— Position  of  various  parts  of  alimentary  canal  at  different  stages.  A.  Em- 
bryo of  five  weeks.  B.  Of  eight  weeks.  C.  Of  ten  weeks  (Allen  "Thomson).  I, 
pharynx  with  the  lungs;  s,  stomach;  i,  small  intestine;  i',  large  intestine;  g,  geni- 
tal duct;  u,  bladder;  cl,  cloaca;  c,  caecum;  vi,  ductus  vitello-intestinalis;  si,  uro- 
genital sinus;  v,  yelk-sac. 


280  COMPARATIVE   PHYSIOLOGY. 

and  hypoblast,  it  is  easy  to  understand  why  the  developed  tract 
should  so  invariaably  consist  of  glandular  structures  and  mus- 
cular tissue  disposed  in  a  certain  regular  arrangement.  The 
fact  that  all  the  organs  that  pour  digestive  juices  into  the  ali- 
mentary tract  are  outgrowths  from  it  serves  to  explain  why 
there  should  remain  a  physiological  connection  with  an  ana- 
tomical isolation.  The  general  resemblance  of  the  epithelium 
throughout,  even  in  parts  widely  separated,  also  becomes  clear, 
as  well  as  many  other  points  we  can  not  now  refer  to  in  detail, 
to  one  who  realizes  the  significance  of  the  laws  of  descent  (evo- 
lution). 

Comparative.  —  Amoeba  ingests  and  digests  apparently  by 
every  part  of  its  body  ;  though  exact  studies  have  shown  that 
it  neither  accepts  nor  retains  without  considerable  power  of 
discrimination  ;  and  it  is  also  possible  that  some  sort  of  digest- 
ive fluid  may  be  secreted  from  the  part  of  the  body  with  which 
the  food-particles  come  in  contact.  It  has  been  shown,  too, 
that  there  are  differences  in  the  digestive  capacity  of  closely 
allied  forms  among  Infusorians. 

The  ciliated  Infusorians  have  a  permanent  mouth,  which 
may  also  serve  as  an  anus  ;  or,  there  may  be  an  anus,  though 
usually  less  distinct  from  the  rest  of  the  body  than  the  mouth. 

Among  the  Ccelenterates  intra-cellular  digestion  is  found. 
Certain  cells  of  the  endoderm  (as  in  Hydra)  take  up  food-parti- 
cles Amceba-like,  digest  them,  and  thus  provide  material  for 
other  cells  as  well  as  themselves,  in  a  form  suitable  for  assimi- 
lation. This  is  a  beginning  of  that  differentiation  of  function 
which  is  carried  so  far  among  the  higher  vertebrates.  But,  as 
recent  investigations  have  shown,  such  intra-cellular  digestion 
exists  to  some  extent  in  the  alimentary  canal  of  the  highest 
members  of  the  vertebrate  group  (see  page  345). 

The  means  for  grasping  and  triturating  food  among  in- 
vertebrates are  very  complicated  and  varied,  as  are  also  those 
adapted  for  sucking  the  juices  of  prey.  Examples  to  hand  are 
to  be  found  in  the  crab,  crayfish,  spider,  grasshopper,  beetle, 
etc.,  on  the  one  hand,  and  the  butterfly,  housefly,  leech,  etc.,  on 
the  other. 

Before  passing  on  to  higher  groups,  it  will  be  well  to  bear 
in  mind  that  the  digestive  organs  are  to  be  regarded  as  the  out- 
come both  of  heredity  and  adaptation  to  circumstances.  We 
find  parts  of  the  intestine,  e.  g.,  retained  in  some  animals  in 
whose  economy  they  seem  to  serve  little  if  any  good  purpose,  as 


DIGESTION   OF    FOOD. 


281 


the  vermiform  appendix  of  man.     Adaptation  has  been  illus- 
trated in  the  lifetime  of  a  single  individual  in  a  remarkable 


Fig.  228. — Diagram  illustrating  arrangement  of  intestine,  nervous  system,  etc.,  in  com- 
mon snail,  Helix  (after  Huxley),  m,  mouth;  t,  tooth;  od,  odontophore;  g,  gullet; 
c,  crop;  a',  stomach;  r,  rectum;  a,  anus;  r.  s,  renal  sac;  h,  heart;  I,  lung  (modified 
pallia)  chamber);  n.  its  external  aperture;  em.  thick  edge  of  mantle  united  with 
sides  of  body;  /,  foot;  cpg,  cerebral,  pedal,  and  parieto-splanchnic  ganglia  aggre- 
gated round  gullet. 

manner  ;  thus,  a  seagull,  by  being  fed  on  grain,  has  had  its 
stomach,  naturally  thin  and  soft-walled,  converted  into  a  mus- 
cular gizzard. 

Since  digestion  is  a  process  in  which  the  mechanical  and 
chemical  are  both  involved,  and  the  food  of  animals  differs  so 
widely,  great  variety  in  the  alimentary  tract,  both  anatomical 
and  physiological,  must  be  expected.  Vegetable  food  must 
usually  be  eaten  in  much  larger  bulk  to  furnish  the  needed 
elements;  hence  the  great  length  of  intestine  habitually  found 
in  herbivorous  animals,  associated  often  with  a  capacious 
and  chambered  stomach,  furnishing  a  larger  laboratory  in 
which  Nature  may  carry  on  her  processes.  To  illustrate,  the 
stomach  of  the  ruminants  consists  of  four  parts  (rumen,  reticu- 
lum, omasum  or  psalterium,  abomasum).  The  food  when 
cropped  is  immediately  swallowed;  so  that  the  paunch  (rumen) 
is  a  mere  storehouse  in  which  it  is  softened,  though  but  little 
changed  otherwise ;  and  it  would  seem  that  real  gastric  digestion 
is  almost  confined  to  the  last  division,  which  may  be  compared 


282 


COMPARATIVE   PHYSIOLOGY. 


to  the  simple  stomach,  of  the  Carnivora  or  of  man  ;  and,  before 
the  food  reaches  this  region,  it  has  been  thoroughly  masticated 
and  mixed  with  saliva. 

The  stomach  of  the  horse  is  small,  though  the  intestine, 


Fig.  229.— The  viscera  of  a  rabbit,  as  seen  upon  simply  opening  the  cavities  of  the 
thorax  and  abdomen  without  any  further  dissection.  A,  cavity  of  the  thorax, 
pleural  cavity  on  either  side;  fi,  diaphragm;  C,  ventricles  of  the  heart;  Z>,  auri- 
cles; E,  pulmonary  artery;  F,  aorta;  G.  lungs  collapsed,  and  occupying  only  back 
part,  of  chest;  //,  lateral  portions  of  pleural  membranes;  /.cartilage  at.  the  end 
of  sternum  (ensiform  cartilage);  K,  portion  of  the  wall  of  body  left  between  thorax 
and  abdomen;  «,  cut  ends  of  the  ribs;  L,  the  liver,  in  this  case  lying  more  to  the 
left  than  to  the  right  of  the  body;  M,  the  stomach,  a  large  part  of  the  greater 
curvature  beting  shown;  IV,  duodenum;  O.  small  intestine;  P,  the  cfecum,  so 
largely  developed  in  this  and  other  herbivorous  animals;  Q,  the  large  intestine. 
'Huxley.) 


DIGESTION   OF   FOOD. 


283 


especially  the  lai'ge  gut  is  capacious.  The  stomach  is  divisible 
into  a  cardiac  region,  of  a  light  color  internally,  and  lined 
with  epithelium,  like  that  of  the   oesophagus,  and  a  redder 


Pig.  230. 


Fig.  232. 


Fig.  230. — General  and  lateral  view  of  dog's  teeth  (after  Chauveau). 

Fig.  231.— Anterior  view  of  incisors  and  canine  teeth  in  a  year-old  dog  (Chauveau). 

Fig.  232.— Dentition  of  inferior  jaw  of  horse  (after  Chauveau). 

pyloric  area,  in  which  the  greater  part  of  the  digestive  process 
goes  on  (Fig.  266). 


284 


COMPARATIVE   PHYSIOLOGY. 


The  mouth  parts,  even  in  some  of  the  higher  vertebrates,  as 
the  Carnivora,  serve  a  prehensile  rather  than  a  digestive  pur- 
pose. This  is  well  seen  in  the  dog,  that  bolts  his  food  ;  but 
in  this  and  allied  groups  of  mammals  gastric  digestion  is  very 
active. 

Tbe  teeth  as  triturating  organs  find  their  highest  develop- 
ment in  ruminants,  the  combined  side-to-side  and  forward-and- 
backward  motion  of  the  jaws  rendering  them  very  effective. 

In  Carnivora  the  teeth  serve  for  grasping  and  tearing,  while 
in  the  Insectivora  the  tongue,  as  also  in  certain  birds  (wood- 
peckers), is  an  important  organ  for  securing  food. 


Fig.  333. — Profile  of  upper  teeth  of  the  horse,  more  especially  intended  to  show  the 
molars,  the  fangs  having  been  exposed  (Chauveau).  a,  molar  teeth;  b,  supple- 
mentary molar;  c,  tusk;  d.  incisors. 

It  is  to  be  noted,  too,  that,  while  the  horse  crops  grass  by 
biting  it  off,  the  ox  uses  the  tongue,  as  well  as  the  teeth  and 
lips,  to  secure  the  mouthful. 

Man's  teeth  are  somewhat  intermediate  in  form  between  the 
carnivorous  and  the  herbivorous  type.  Birds  lack  teeth,  but 
the  strong  muscular  gizzard  suffices  to  grind  the  food  against 
the  small  pebbles  that  are  habitually  swallowed. 

The  crop,  well  developed  in  granivorous  birds,  is  a  dilatation 
of  the  oesophagus,  serving  to  store  and  soften  the  food. 

In  the  pigeon  a  glandular  epithelium  in  the  crop  secretes  a 

Fig.  234.— General  view  of  digestive  apparatus  of  fowl  (after  Chauveau).  1,  tongue; 
2  pharynx;  3,  first  portion  of  oesophagus;  4,  crop;  5,  second  portion  of  oesopha- 
gus; 6,  succentric  ventricle  (proventricuniB);  7,  gizzard;  8,  origin  of  duodenum;  !), 
first  branch  of  duodenal  flexure;  10,  second  branch  of  same;  11,  origin  of  floating 
portion  of  small  intestine;  12,  small  intestine;  12',  terminal  portion  of  this  intes- 
tine, Hanked  on  each  side  by  the  two  coeca  (regarded  as  the  analogue  of  colon  of 
mammals);  18,  13,  free  extremities  of  csecums;  11.  insertion  of  these  two  culs-de- 
sac  into  intestinal  tube;  15,  rectum;  lfi,  cloaca;  17',  anus;  18,  mesentery;  19.  left 
lobe  ol'  liver;  20,  right,  lobe;  21,  gall-bladder;  22,  insertion  of  pancreatic  and  biliary 
ducts;  the  two  pancreatic  ducts  are  the  most  anterior,  the  choledic  or  hepatic  is  in 
the  middle,  and  the  cystic  duct  is  posterior;  23,  pancreas;  24,  diaphragmatic  aspect 
of  lung;  25,  ovary  (in  a  state  of  atrophy);  2G,  oviduct. 


Pia.  234. 


286 


COMPARATIVE  PHYSIOLOGY. 


milky-looking  substance  that  is  regurgitated  into  the  mouth  of 
the  young  one,  which  is  inserted  within  that  of  the  parent  bird. 

The  proventriculus — an  enlargement  just  above  the  gizzard 
—is  relatively  to  the  latter  very  thin -walled,  but  provides  the 
true  gastric  juices. 

Certain  plants  digest  proteid  matter,  like  animals  ;  thus  the 
sun-dew  (Drosera),  by  the  closure  of  its  leaves,  captures  insects, 
which  are  digested  and  the  products  absorbed.  The  digestive 
fluid  consists  of  a  pepsin-containing  secretion,  together  with 
formic  acid. 


STRUCTURE,   ARRANGEMENT,   AND    SIGNIFICANCE 
OF   THE   TEETH. 

In  a  tooth  we  recognize  a  portion  imbedded  in  the  jaw  (fang, 
root),  a  free  portion  (crown),  and  a  constricted  region  (neck). 

B 


Ki...  ;r,:, 


Fig.  237. 


Pia 


835— Magnified  section  of  a  canine  tooth,  showing  its  intimate  structure.  1, 
crown-  2  -i  neck-  3  fang,  or  root;  4,  cavitas  pulpae;  5,  opening  by  which  the  ves- 
sels and  nerves  communicate  with  the  pulp;  6,  6,  ivory,  showing  fibrous  structure; 
7,7,  enamel;  8,  8,  cement.  ,  . 

Pio.  236.— A,  transverse  section  of  enamel,  Showing  its  hexagonal  prisms;  B,  sepa- 
rated prisms  (Cliauveau). 

Kir.  !.':{7  Section  through  fang  of  molar  tooth  (Cliauveau).  a,  a.  dentine  traversed 
by  its  tubuli;  b,  b,  interglobular  or  nodular  layer;  c,  c,  cementum. 


DIGESTION  OF   POOD.  287 


Pig.  238.— Incisor  teeth  of  the  horse.  Details  of  structure  (Chauveau).  1,  a  tooth  in 
which  is  indicated  general  shape  of  a  permanent  incisor  and  the  particular  forms 
successively  assumed  by  dental  table  in  consequence  of  friction  and  the  continued 
pushing  outward  of  these  teeth;  2.  a  virgin  tooth,  anterior  and  posterior  faces;  3, 
longitudinal  section  of  a  virgin  tooth,  intended  to  show  the  internal  conformation 
and' structure.  Not  to  complicate  the  figure,  the  external  cement  and  that  amassed 
in  the  infundibulum  have  not  been  exhibited;  4,  transverse  section  for  the  same 
purpose;  a,  encircling  enamel;  b,  central  enamel;  c,  dental  star;  d,  dentine;  5,  de- 
ciduous tooth. 

A  tooth  is  made  up  of  enamel,  dentine  or  "  ivory,"  and  ce- 
ment (crusta  petrosa).  The  relative  distribution  of  these  is 
shown  in  Fig.  235. 


B  D 


Fig.  239.— Transverse  section  of  a  horse's  upper  molar  tooth  (Chauveau).    A,  external 
cement;  JB,  external  enamel;  C,  dentine;  D,  internal  enamel;  E,  internal  cement. 


288 


COMPARATIVE  PHYSIOLOGY. 


Enamel  is  made  up  of  elongated  hexagonal  prisms  set  almost 
vertically  in  the  dentine  (Fig.  236). 

It  is  the  hardest  substance  known  in  the  animal  body,  con- 
sisting almost  entirely  of  inorganic  material  ;  and  when  lost  is 
but  indifferently  if  at  all  replaced. 


Fio.  240.— Tooth  of  cat  in  situ  (Waldeyer).    1,  enamel;  2,  dentine;  3,  cement;  4,  peri- 
osteum of  alveolar  cavity;  5,  bone  of  jaw;  6,  pulp  cavity. 

Dentine   is    traversed    by   the   dentine  tubules    (Fig.   237), 
which  radiate  outward  from  the  pulp  cavity. 


DIGESTION   OF   FOOD. 


289 


The  latter  is  filled  by  the  tooth-pulp,  which  consists  of  a 
delicate  connective  tissue  supporting  blood-vessels  and  nerves 
which  ramify  in  it  after  entering  by  the  openings  in  the  fang 
of  the  tooth.  From  the  pulp  protoplasmic  fibers  extend  into 
the  dentine  tubules. 

The  crusta  petrosa  is  very  similar  to  bone,  but  is  usually 
without  Haversian  canals,  and,  like  bone,  is  covered  with  peri- 
osteum. 


Fig.  241.— The  teeth  of  the  ox  (Chauveau).     1,  upper  jaw,  with  a,  friction  surface  and 
b,  external  surface;  2,  lower  jaw,  with  0,  dental  tables,  and  b,  external  face.  ' 

Teeth  are  simple  and  compound.     In  the  former  (carnivora) 
the  entire  crown  is  covered  with  enamel  ;  in  the  latter,  owing 
19 


290 


COMPARATIVE   PHYSIOLOGY. 


to  wear,  the  other  constituents  appear  on  the  upper  surface  of 
the  crown  (Figs.  238,  239,  240). 

It  follows  that  the  former  are  better  adapted  for  tearing, 
the  latter  for  grinding,  as  the  different  components  wear  un- 
equally and  leave  the  top  of  the  crown  rough,  so  that  the  upper 
and  lower  jaws  of  a  ruminant  are  like  two  millstones,  (Fig. 
241). 

It  also  follows  that  in  the  horse  and  in  ruminants  the  age 
may  be  learned  with  considerable  accuracy  from  the  condition 
of  wear  of  the  teeth  and  as  the  incisors  are  most  readily  ex- 
amined they  are  taken  as  the  chief  indicators  of  the  age  of 
the  animal. 

In  nearly  all  animals  are  found  the  deciduous  or  milk  teeth 
succeeded  by  the  permanent  teeth.  This  arises  as  a  necessity 
from  the  growth  of  the  jaw  and  the  need  of  stronger  teeth,  either 
as  weapons  of  defense  and  attack  or  in  order  the  more  effectu- 
ally to  secure  and  prepare  food.  The  permanent  teeth  are  also 
more  numerous  than  the  milk  teeth. 

The  dentition  of  our  domestic  animals  may  be  expressed 
thus  : 


Dos. 


Cat. 


Man. 


Pig- 


Ox. 


Horse. 


.         3-3 
Incisors,  o~~q 

1-1 
canines,  y~. r 

3-3 

1-1 

3-3 

1-1 

2-2 

1-1 

2-2 

1-1 

3-3 

1-1 

3-3 

1-1 

0-0 

0-0 

3-3 

1-1 

3-3 

1-1 

3-3 

1-1 

3-3 

0-0 

3-3 

0-0 

4-4 
premolars,  t^7 

3-3 
2-2 
2-2 
2-2 
3-3 
3-3 
3-3 
3-3 
3-3 
3-3 
0-0 
0-0 


2-2 

molars,  5—5  =  42. 


1-1 
1-1 
3-3 
3-3 
4-4 
4-4 
3-3 
3-3 
3-3 
3-3 
3-3 
3-3 


=  30. 


=  32. 


=  44. 


32. 


=  40. 


=  24. 


The  latter  is  the  representation  of  the  milk  dentition.  The 
mare  is  without  canines  ("tushes"). 

It  will  be  noticed  that  in  the  ox  incisors  and  canines  do  not 
appear  in  the  upper  jaw,  though  they  are  represented  by  embry- 
onic rudiments. 

The  table  above  and  that  on  page  296  (after  Leyh)  give  a 
large  amount  of  information  in  a  small  space,  and  are  illus- 
trated by  the  accompanying  figures  : 


DIGESTION   OF   FOOD. 


291 


Fig.  242.— The  teeth  of  the  pig  (Chauveau).    1,  upper  teeth,  table  surface;  2,  lower 
teeth,  table  aspect;  3,  lateral  view  of  jaws. 


2$  years.  4  years. 

(6  broad  incisors.)  (8  broad  incisors.) 


Over  7  years, 
broad  incisors.) 


2  months. 
(Milk-teeth.) 


1±  years. 
(2  broad  incisors.) 


If  years. 
(4  broad  incisors.) 


Fig.  243. — Changes  in  incisor  teeth  of  the  sheep  (WilckensX 


292 


COMPARATIVE  PHYSIOLOGY. 


New-born. 


3  months. 


6  months 


JSwSl^ 


■"■'■"■■  "  ■ '  ■ 


1  year. 


2  years. 


2£  years. 


3  years. 


4  years. 


5  years. 


(i  years.  7  years. 

Fig.  244  (1).— Changes  in  incisor  teeth  of  horse  with  age  (Wilckens). 


DIGESTION    OF   POOD 


293 


8  years. 


it  years. 


12  years. 


15  years. 


18  years.  24  years. 

Fig.  244  (2).  Changes  in  incisor  teeth  of  horse  with  age  (Wilckens). 


294 


COMPARATIVE   PHYSIOLOGY. 


1 
New-born. 


JSD 


4  weeks. 


1  year. 


2  years. 


2|  years. 


3  years. 


•3J  years. 


4  years.  5  years. 

Fig.  245  (1).— Changes  in  incisor  teeth  of  ox  with  age  (Wilckens). 


DIGESTION   OF   POOD. 


295 


6  years. 


'  years. 


8  years. 


10  years. 


12  years. 


14  years. 


16  years.  18  years.  20  years. 

Fig.  245  (2).— Changes  in  incisor  teeth  of  ox  with  age  (Wilckens). 


296 


COMPARATIVE  PHYSIOLOGY. 


a 

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DIGESTION  OP  FOOD.  297 


THE   DIGESTIVE   JUICES. 

Saliva. — The  saliva  as  found  in  the  mouth  is  a  mixture  of 
the  secretion  of  three  pairs  of  glands,  alkaline  in  reaction,  of  a 
low  specific  gravity  (variable  in  different  groups  of  animals), 
with  a  small  percentage  of  solids  consisting  of  salts  and  organic 
bodies  (mucin,  proteids). 

Saliva  serves  mechanical  functions  in  articulation,  in  moist- 
ening the  food,  and  dissolving  out  some  of  its  salts.  But  its 
principal  use  in  digestion  is  in  reducing  starchy  matters  to  a 
soluble  form,  as  sugar.  So  far  as  known,  the  other  constituents 
of  the  food  are  not  changed  chemically  in  the  mouth. 

The  Amylolytic  Action  of  the  Saliva.— Starch  exists  in  grains 
surrounded  by  a  cellulose  covering,  which  saliva  does  not  digest : 
hence  its  action  on  raw  starch  is  slow. 

It  is  found  that  if  a  specimen  of  boiled  starch  not  too  thick 
be  exposed  to  a  small  quantity  of  saliva  at  the  temperature  of 
the  body  or  thereabout  (37°  to  40°  C),  it  will  speedily  undergo 
certain  changes  : 

1.  After  a  very  short  time  sugar  may  be  detected  by  Feh- 
ling's  solution  (copper  sulphate  in  an  excess  of  sodium  hydrate, 
the  sugar  reducing  the  cupric  hydrate  to  cuprous  oxide  on 
boiling). 

2.  At  this  early  stage  starch  may  still  be  detected  by  the 
blue  color  it  gives  with  iodine  ;  but  later,  instead  of  a  blue,  a 
purple  or  red  may  appear,  indicating  the  presence  of  dextrin, 
which  may  be  regarded  as"  a  product  intermediate  between 
starch  and  sugar. 

3.  The  longer  the  process  continues,  the  more  sugar  and  the 
less  starch  or  dextrin  to  be  detected  ;  but,  inasmuch  as  the 
quantity  of  sugar  at  the  end  of  the  process  does  not  exactly 
correspond  with  the  original  quantity  of  starch,  even  when  no 
starch  or  dextrin  is  to  be  found,  it  is  believed  that  other  bodies 
are  formed.  One  of  these  is  achroodextrin,  which  does  not 
give  a  color  reaction  with  iodine. 

The  sugars  formed  are:  (a)  Dextrose.  (6)  Maltose,  which 
has  less  reducing  power  over  solutions  of  copper  salts,  a  more 
pronounced  rotatory  action  on  light,  etc. 

It  is  found  that  the  digestive  action  of  saliva,  as  in  the 
above-described  experiment,  will  be  retarded  or  arrested  if  the 
sugar  is  allowed  to  accumulate  in  large  quantity.  That  diges- 
tion in  the  mouth  is  substantially  the  same  as  that  just  de- 


298  COMPARATIVE   PHYSIOLOGY. 

scribed  can  be  easily  sbown  by  holding  a  solution  of  starch  in 
the  mouth  for  a  few  seconds,  and  then  testing  it  for  sugar, 
when  it  will  be  invariably  found. 

While  salivary  digestion  is  not  impossible  in  a  neutral  me- 
dium, it  is  arrested  in  an  acid  one  even  of  no  great  strength 
(less  than  one  per  cent),  and  goes  on  best  in  a  feebly  alkaline 
medium,  which  is  the  condition  normally  in  the  mouth.  Though 
a  temperature  about  equal  to  that  of  the  body  is  best  adapted 
for  salivary  digestion,  it  will  proceed,  we  have  ourselves  found 
at  a  higher  temperature  than  digestion  by  any  other  of  the 
juices,  so  far  as  man  is  concerned — a  fact  to  be  connected,  in  all 
probability,  with  his  habit  for  ages  of  taking  very  warm  fluids 
into  the  mouth. 

The  active  principle  of  saliva  is  ptyalin,  a  nitrogenous  body 
which  is  assumed  to  exist,  for  it  has  never  been  perfectly  iso- 
lated. It  belongs  to  the  class  of  unorganized  ferments,  the 
properties  of  which  have  been  already  referred  to  before  (page 
162). 

Characteristics  of  the  Secretion  of  the  Different  Glands  — 
Parotid  saliva  is  in  man  not  a  viscid  fluid,  but  clear  and  limpid, 
containing  very  little  mucin.  Submaxillary  saliva  in  most 
animals  and  in  man  is  viscid,  while  the  secretion  of  the  sub- 
lingual gland  is  still  more  viscid. 

Comparative. — Saliva  differs  greatly  in  activity  in  different 
animals  ;  thus  saliva  in  the  dog  is  almost  inert,  that  of  the 
parotid  gland  quite  so ;  in  the  cat  it  is  but  little  more  effective ; 
and  in  the  horse,  ox,  and  sheep,  it  is  known  to  be  of  very  feeble 
digestive  power. 

In  man,  the  Guinea-pig,  the  rat,  the  hog,  both  parotid  and 
submaxillary  saliva  are  active  ;  while  in  the  rabbit  the  sub- 
maxillary saliva,  the  reverse  of  the  preceding,  is  almost  in- 
active, and  the  parotid  secretion  very  powerful. 

An  aqueous  or  glycerin  extract  of  the  salivary  glands  has 
digestive  properties.  The  secretion  of  the  different  glands  may 
be  collected  by  passing  tubes  or  cannulas  into  their  ducts. 

The  saliva,  normally  neutral  or  only  faintly  acid,  may  be- 
come very  much  so  in  the  intervals  of  digestion.  The  rapid 
decay  of  the  teeth  occurring  during  and  after  certain  diseases 
seems  in  certain  cases  to  be  referable  in  part  to  an  abnormal 
condition  of  the  saliva. 

The  tartar  which  collects  on  the  teeth  consists  largely  of 
earthy  phosphates. 


DIGESTION   OP   FOOD.  299 

Gastric  Juice. — Gastric  juice  may  be  obtained  from  a  fistu- 
lous opening  into  the  stomach.  Such  may  be  made  artificially 
by  an  incision  over  the  organ  in  the  middle  line,  catching  it  up 
and  stitching  it  to  the  edges  of  the  wound,  incising  and  insert- 
ing a  special  form  of  cannula,  which  may  be  closed  or  opened 
at  will. 

Digestion  in  a  few  cases  of  accidental  gastric  fistulas  has 
been  made  the  subject  of  careful  study.  The  most  instructive 
case  is  that  of  Alexis  St.  Martin,  a  French  Canadian,  into 
whose  stomach  a  considerable  opening  was  made  by  a  gunshot- 
wound. 

Gastric  juice,  in  his  case  and  in  the  lower  animals  with 
artificial  openings  in  the  stomach,  has  been  obtained  by  irri- 
tating the  mucous  lining  mechanically  with  a  foreign  body,  as 
a  feather. 

The  great  difficulty  in  all  such  cases  ai^ises  from  the  impos- 
sibility of  being  certain  that  such  fluid  is  normal ;  for  the  con- 
ditions which  call  forth  secretion  are  certainly  such  as  the 
stomach  never  experiences  in  the  ordinary  course  of  events, 
and  we  have  seen  how  saliva  vaiies,  according  as  the  animal  is 
fasting  or  feeding,  etc. 

Bearing  in  mind,  then,  that  our  knowledge  is  possibly  only 
approximately  correct,  we  may  state  what  is  known  of  the  se- 
cretions of  the  stomach. 

The  gastric  secretion  is  clear,  colorless,  of  low  specific  grav- 
ity (lOOl  to  1010),  the  solids  being  in  great  part  made  up  of 
pepsin  with  a  small  quantity  of  mucus,  which  may  become  ex- 
cessive in  disordered  conditions.  There  has  been  a  good  deal 
of  dispute  as  to  the  acid  found  in  the  stomach  during  digestion. 
It  is  now  generally  agreed  that  during  the  greater  part  of  the 
digestive  process  there  is  free  hydrochloric  acid  to  the  extent 
of  about  "2  per  cent.  It  is  maintained  that  lactic  acid  exists 
normally  in  the  early  stages  of  digestion,  and  it  is  conceded  that 
lactic,  butyric,  acetic,  and  other  acids  may  be  present  in  certain 
forms  of  disordered  digestion. 

It  is  also  generally  acknowledged  that  in  mammals  the  work 
of  the  stomach  is  limited,  so  far  as  actual  chemical  changes  go, 
to  the  conversion  of  the  proteid  constituents  of  food  into  pep- 
tone. Fats  maybe  released  from  their  proteid  coverings  (cells), 
but  neither  they  nor  starches  are  in  the  least  altered  chemically. 
Some  have  thought  that  in  the  dog  there  is  a  slight  digestion  of 
fats  in  the  stomach.     The  solvent  power  of  the  gastric  juice  is 


300  COMPARATIVE   PHYSIOLOGY. 

greater  than  can  be  accounted  for  by  the  presence  of  the  acid  it 
contains  merely,  and  it  has  a  marked  antiseptic  action. 

Digestive  processes  may  be  conducted  out  of  the  body  in  a 
very  simple  manner,  which  the  student  may  carry  out  for  him- 
self. To  illustrate  by  the  case  of  gastric  digestion :  The  mucous 
membrane  is  to  be  removed  from  a  pig's  stomach  after  its  sur- 
face has  been  washed  clean,  but  not  too  thoroughly,  chopped 
up  fine,  and  divided  into  two  parts.  On  one  half  pour  water 
that  shall  contain  "2  per  cent  hydrochloric  acid  (made  by  add- 
ing 4  to  6  cc.  commercial  acid  to  1,000  cc.  water).  This  will 
extract  the  pepsin,  and  may  be  used  as  the  menstruum  in  which 
the  substance  to  be  digested  is  placed.  The  best  is  fresh  fibrin 
whipped  from  blood  recently  shed. 

Since  the  fluid  thus  prepared  will  contain  traces  of  peptone 
from  the  digestion  of  the  mucous  membrane,  it  is  in  some 
respects  better  to  use  a  glycerin  extract  of  the  same.  This  is 
made  by  adding  some  of  the  best  glycerin  to  the  chopped  up 
mucous  membrane  of  the  stomach  of  a  pig,  etc.,  well  dried  with 
bibulous  paper,  letting  the  whole  stand  for  eight  to  ten  days, 
filtering  through  cotton,  and  then  through  coarse  filter-paper. 
It  will  be  nearly  colorless,  clear,  and  powerful,  a  few  drops 
sufficing  for  the  work  of  digesting  a  little  fibrin  when  added  to 
some  two  per  cent  of  hydrochloric  acid. 

Digestion  goes  on  best  at  about  40°  C,  but  will  proceed  in 
the  cold  if  the  tube  in  which  the  materials  have  been  placed  is 
frequently  shaken.  It  is  best  to  place  the  test-tube  containing 
them  in  a  beaker  of  water  kept  at  about  blood-heat.  Soon  the 
fibrin  begins  to  swell  and  also  to  melt  away. 

After  fifteen  to  twenty  minutes,  if  a  little  of  the  fluid  in  the 
tube  be  removed  and  filtered,  and  to  the  filtrate  added  carefully 
to  neutralization  dilute  alkali,  a  precipitate,  insoluble  in  water 
but  soluble  in  excess  of  alkali  (or  acid),  is  thrown  down.  This 
is  in  most  respects  like  acid-albumen,  but  has  been  called  para- 
peptone.  The  longer  digestion  proceeds,  the  less  is  there  of 
this  and  the  more  of  another  substance,  peptone,  so  that  the 
former  is  to  be  regarded  as  an  intermediate  product.  Peptone 
is  distinguished  from  albuminous  bodies  or  proteids  by — 1. 
Not  being  coagtilable  from  its  aqueous  solutions  on  boiling. 
2.  Diffusing  more  readily  through  animal  membranes.  3.  Not 
being  precipitated  by  a  number  of  reagents  that  usually  act 
on  proteids. 

In  artificial  digestion  it  is  noticeable  that  much  more  fibrin 


DIGESTION   OP   FOOD. 


301 


or  other  proteid  matter  will  be  dissolved  if  it  be  finely  divided 
and  frequently  shaken  up,  so  that  a  greater  surface  is  exposed 
to  the  digestive  fluid. 

The  exact  nature  of  the  process  by  which  proteid  is  changed 
to  peptone  is  not  certainly  known. 

Since  starch  on  the  addition  of  water  becomes  sugar  (C6Hi0 
05  +  H20  =  CoHjoOo),  and  since  peptones  have  been  formed 
through  the  action  of  dilute  acid  at  a  high  temperature  or  by 
superheated  water  alone,  it  is  possible  that  the  digestion  of  both 
starch  and  proteids  may  be  a  hydration  ;  but  we  do  not  know 
that  it  is  such. 

As  already  explained,  milk  is  curdled  by  an  extract  of  the 
stomach  (rennet) ;  and  this  can  take  place  in  the  absence  of  all 
acids  or  anything  else  that  migbt  be  suspected  except  the  real 
cause ;  tbere  seems  to  be  no  doubt  that  there  is  a  distinct  fer- 
ment which  -  produces  the  coagulation  of  milk  which  results 
from  the  precipitation  of  its  casein. 

The  activity  of  the  gastric  juice,  and  all  extracts  of  the  mu- 
cous membrane  of  the  stomach,  on  proteids,  is  due  to  pepsin,  a 
nitrogenous  body,  but  not  a  proteid. 

Like  other  ferments,  the  conditions  under  which  it  is  effect- 
ive are  well  defined.  It  will  not  act  in  an  alkaline  medium  at 
all,  and  if  kept  long  in  such  it  is  destroyed.  In  a  neutral  me- 
dium its  power  is  suspended  but  not  destroyed.  Digestion  will 
go  on,  though  less  perfectly,  in  the  presence  of  certain  other 
acids  than  hydrochloric.  As  with  all  digestive  ferments,  the 
activity  of  pepsin  is  wholly  destroyed  by  boiling. 


IN  100  PARTS. 

Man. 

Ox. 

Pig. 

DOG. 

Fresh. 

From  gall- 
bladder. 

Water 

86-3 
13-7 

7-4 

3-0 
2-2 
1-1 

90-4 
.  9-6 

[■   8-0 

0-3 
1-3 

ss-s 

11-2 

|    7-3 

S    2-2 
0-6 
1-1 

95-3 

4-7 
3-4 

0-5 
0-2 

0-6 

85-2 

Solids 

14-8 

Bile  salts 

12-6 

Fats,  soaps 

1-3 

Mucin  and  coloring  matter  . . 
Inorganic  salts 

0-3 

0-6 

The  color  of  the  bile  of  man  is  a  rich  goldeu  yellow.  When 
it  contains  much  mucus,  as  is  the  case  when  it  remains  long  in 
the  gall-bladder,  it  is  ropy,  though  usually  clear.  Bile  may 
contain  small  quantities  of  iron,  manganese,  and  copper,  the 


302  COMPARATIVE   PHYSIOLOGY. 

latter  two  especially  being  absent  from  all  other  fluids  of  the 
body.  Sodium  chloride  is  the  most  abundant  salt.  Bile  must 
be  regarded  as  an  excretion  as  well  as  a  secretion  ;  the  pig- 
ments, copper,  manganese,  and  perhaps  the  iron  and  the  cho- 
lesterin  being  of  little  or  no  use  in  the  digestive  processes,  so 
far  as  known. 

The  bile-salts  are  the  essential  constituents  of  bile  as  a 
digestive  fluid.  In  man  and  many  other  animals,  they  consist 
of  taurocholate  and  glycocholate  of  sodium,  and  may  be  ob- 
tained in  bundles  of  needle-shaped  crystals  radiating  from  a 
common  center.  These  salts  are  soluble  in  water  and  alcohol, 
with  an  alkaline  reaction,  but  insoluble  in  ether. 

Glycocholic  acid  may  be  resolved  into  cholalic  (cholic)  acid 
and  glycin  (glycocol)  ;  and  taurocholic  acid  into  cholalic  acid 
and  taurin.     Thus  : 

Glycocholic  acid.  Cholalic  acid.  Glycin. 

C26H43N08  +  H20  =  C24H40O5  +  C2H*N02. 

Taurocholic  acid.  Cholalic  acid.  Taurin. 

CseBUsNSO,  +  H20  =  C24H40O6  +  C2H7NS03. 


Glycocol  (glycin)  is  amido-acetic  acid 

CH2<( 
Taurin,  amido-isethionic  acid, 


CH<co!k'and 


C2H4<tstt|  ,  and  may  be  made  artifi- 
cially from  isethionic  acid. 

It  is  to  be  noted  that  both  the  bile  acids  contain  nitrogen, 
but  that  cholalic  acid  does  not.  The  decomposition  of  the  bile 
acids  takes  place  in  the  alimentary  canal,  and  the  glycin  and 
taurin  are  restored  to  the  blood,  and  are  possibly  used  afresh  in 
the  construction  of  the  bile  acids,  though  this  is  not  definitely 
known. 

Bile-Pigments. — The  yellowish-red  color  of  the  bile  is  owing 
to  Bilirubin  (CioHmNaOa),  which  may  be  separated  either  as 
an  amorphous  yellow  powder  or  in  tablets  and  prisms.  It  is 
soluble  in  chloroform,  insoluble  in  water,  and  but  partially 
soluble  in  alcohol  and  ether.  It  makes  up  a  large  part  of 
gall-stones,  which  contain,  besides  cholesterin,  earthy  salts  in 
abundance. 

It  may  be  oxidized  to  Biliverdin  (CmHisN^),  the  natural 
green  pigment  of  the  bile  of  the  herbivora.     When  a  drop  of 


DIGESTION  OP   FOOD.  303 

nitric  acid,  containing  nitrous  acid,  is  added  to  bile,  it  under- 
goes a  series  of  color  changes  in  a  certain  tolerably  constant 
order,  becoming  green,  greenish-blue,  blue,  violet,  a  brick  red, 
and  finally  yellow  ;  though  the  green  is  the  most  characteristic 
and  permanent.  Each  one  of  these  represents  a  distinct  stage 
of  the  oxidation  of  bilirubin,  the  green  answering  to  biliverdin. 
Such  is  Gmelin's  test  for  bile-pigments,  by  which  they  may  be 
detected  in  urine  or  other  fluids.  The  absence  of  proteids  in 
bile  is  to  be  noted. 

The  Digestive  Action  of  Bile. — 1.  So  far  as  known,  its  action 
on  proteids  is  nil.  When  bile  is  added  to  the  products  of  an 
artificial  gastric  digestion,  bile-salts,  peptone,  pepsin,  and  para- 
peptone  are  precipitated  and  redissolved  by  excess.  2.  It  is 
slightly  solvent  of  fats,  though  an  emulsion  made  with  bile  is 
very  feeble.  But  it  is  likely  helpful  to  pancreatic  juice,  or 
more  efficient  itself  when  the  latter  is  present.  With  free  fatty 
acids  it  forms  soaps,  which  themselves  help  in  emulsifying  fat. 
3.  Membranes  wet  with  bile  allow  fats  to  pass  mere  readily; 
hence  it  is  inferred  that  bile  assists  in  absorption.  4.  When 
bile  is  not  poured  out  into  the  alimentary  canal  the  faeces 
become  clay-colored  and  ill-smelling,  foul  gases  being  secreted 
in  abundance,  so  that  it  would  seem  that  bile  exercises  an  anti- 
septic influence.  It  may  limit  the  quantity  of  indol  formed. 
It  is  to  be  understood  that  these  various  properties  of  bile  are 
to  be  traced  almost  entirely  to  its  salts  ;  though  its  alkaline 
reaction  is  favorable  to  digestion  in  the  intestines,  apart  from 
its  helpfulness  in  soap-forming,  etc.  5.  It  is  thought  by  some 
that  the  bile  acts  as  a  stimulant  to  the  intestinal  tract,  giving 
rise  to  peristaltic  movements,  and  also,  mechanically,  as  a  lubri- 
cant of  the  faeces.  In  the  opinion  of  many,  an  excess  of  bile 
naturally  poured  out  causes  diarrhoea,  and  it  is  well  known 
that  bile  given  by  the  mouth  acts  as  a  purgative.  However, 
we  must  distinguish  between  the  action  of  an  excess  and  that 
of  the  quantity  secreted  by  a  healthy  individual.  The  acid  of 
the  stomach  has  probably  no  effect  allied  to  that  produced  by 
giving  acids  medicinally,  which  warns  us  that  too  much  must 
not  be  made  out  of  the  argument  from  bilious  diarrhsea.  6.  As 
before  intimated,  a  great  part  of  the  bile  must  be  regarded  as 
excrementitious.  It  looks  as  though  much  of  the  effete  haemo- 
globin of  the  blood  and  of  the  cholesterin,  which  represents 
possibly  some  of  the  waste  of  nervous  metabolism,  were  expelled 
from  the  body  by  the  bile.     The  cholalic  acid  of  the  fasces  is 


304 


COMPARATIVE   PHYSIOLOGY. 


derived  from  the  decomposition  of  the  bile  acids.  Part  of  their 
mucus  must  also  be  referred  to  the  bile,  the  quantity  originally 
present  in  this  fluid  depending  much  on  the  length  of  its  stay 
in  the  gall-bladder,  which  secretes  this  substance.  7.  There  is 
throughout  the  entire  alimentary  tract  a  secretion  of  mucus 
which  must  altogether  amount  to  a  large  quantity,  and  it  has 
been  suggested  that  this  has  other  than  lubricating  or  such  like 
functions.  It  appears  that  mucus  may  be  resolved  into  a  pro- 
teid  and  an  animal  gum,  which  latter,  it  is  maintained,  like 
vegetable  gums,  assists  emulsification  of  fats.  If  this  be  true, 
and  the  bile  is,  as  has  been  asserted,  possessed  of  the  power  to 
break  up  this  mucus  (mucin),  its  emulsifying  effect  in  the  in- 
testine may  indirectly  be  considerable.  Bile  certainly  seems  to 
intensify  the  emulsifying  power  of  the  pancreatic  juice. 

There  does  not  seem  to  be  any  ferment  in  bile,  unless  the 
power  to  change  starch  into  sugar,  peculiar  to  this  secretion  in 
some  animals,  is  owing  to  such. 

Comparative. — The  bile  of  the  carnivora  and  omnivora  is 
yellowish-red  in  color;  that  of  herbivora  green.  The  former 
contains  taurocholate  salts  almost  exclusively ;  in  herbivorous 
animals  and  man  there  is  a  mixture  of  the  salts  of  both  acids, 
tbough  tbe  glycocholate  predominates. 


Fig.  240.— Gallbladder,  ductus  choledochus  and  pancreas  in  man  (after  Le  Bon).  _  a, 
gall-bladder:  l>.  hepatic  duct;  c,  opening  of  second  duct  of  pancreas;  <l.  opening 
of  main  pancreatic  duct  and  bile-duct;  e,  e,  duodenum;  /,  ductus  choledochus;  p, 
pancreas. 


DIGESTION  OF   FOOD. 


305 


Pancreatic  Juice. — This  fluid  is  found  to  vary  a  good  deal 
quantitatively,  according  as  it  is  obtained  from  a  temporary 
(freshly  made)  or  permanent  fistula — a  fact  which  emphasizes 
the  necessity  for  caution  in  drawing  conclusions  about  the 
digestive  juices  as  obtained  by  our  present  methods. 

The  freshest  juice  obtainable  through  a  recent  fistulous 
opening  in  the  pancreatic  duct  is  clear,  colorless,  viscid,  alka- 
line in  reaction,  and  with  a  very  variable  quantity  of  solids 
(two  to  ten  per  cent),  less  than  one  per  cent  being  inorganic 
matter. 

Among  the  organic  constituents  the  principal  are  albumin, 
alkali-albumin,  peptone,  leucin,  tyrosin,  fats,  and  soaps  in  small 
amount.     The  alkalinity  of  the  juice  is  owing  chiefly  to  sodium 


Fig.  247.— Crystals  of  leucin  (Funke). 


Fig.  248.— Crystals  of  tyrosin  (Funke). 


carbonates,  which  seem  to  be  associated  with  some  proteid 
body.  There  is  little  doubt  that  leucin,  tyrosin,  and  peptone 
arise  from  digestion  of  the  proteids  of  the  juice  by  its  own 
action. 

Experimental.— If  the  pancreatic  gland  be  mostly  freed  from 
adhering  fat,  cut  up,  and  washed  so  as  to  get  rid  of  blood; 
then  minced  as  fine  as  possible,  and  allowed  to  stand  in  one-per- 
cent sodium-carbonate  solution  at  a  temperature  of  40°  C,  the 
following  results  maybe  noted:  1.  After  a  variable  time  the 
reaction  may  change  to  acid,  owing  to  free  fatty  acid  from 
the  decomposition  (digestion)  of  neutral  fats.  2.  Alkali-albu- 
min, or  a  body  closely  resembling  it,  may  be  detected  and  sep- 
arated by  neutralization.     3.  Peptone  may  be  detected  by  the 

20 


306  COMPARATIVE   PHYSIOLOGY. 

use  of  a  trace  of  copper  sulphate  added  to  a  few  drops  of  caustic 
alkali,  which  becomes  red  if  this  body  be  present.  4.  After  a 
few  hours  the  smell  becomes  faecal,  owing'  in  part  to  indol, 
which  gives  a  violet  color  with  chlorine-water;  while  under 
the  microscope  the  digesting  mass  may  be  seen  to  be  swarming 
with  bacteria.  5.  When  digestion  has  proceeded  for  some  time, 
leucin  and  tyrosin  may  be  shown  to  be  present,  though  their 
satisfactory  separation  in  crystalline  form  involves  somewhat 
elaborate  details.  These  changes  are  owing  to  self-digestion 
of  the  gland. 

All  the  properties  of  this  secretion  may  be  demonstrated 
more  satisfactorily  by  making  an  aqueous  or,  better,  glycerin 
extract  of  the  pancreas  of  an  ox,  pig,  etc.,  and  carrying  on  arti- 
ficial digestion,  as  in  the  case  of  a  peptic  digestion,  with  fibrin. 
In  the  case  of  the  digestion  of  fat,  the  emulsifying  power  of  a 
watery  extract  of  the  gland  may  be  shown  by  shaking  up  a 
little  melted  hog's  lard,  olive-oil  (each  quite  fresh,  so  as  to  show 
no  acid  reaction),  or  soap.  Kept  under  proper  conditions,  free 
acid,  the  result  of  decomposition  of  the  neutral  fats  or  soap 
into  free  acid,  etc.,  may  be  easily  shown.  The  emulsion,  though 
allowed  to  stand  long,  persists,  a  fact  which  is  availed  of  to 
produce  more  palatable  and  easily  assimilated  preparations  of 
cod-liver  oil,  etc.,  for  medicinal  use. 

Starch  is  also  converted  into  sugar  with  great  ease.  In 
short,  the  digestive  juice  of  the  pancreas  is  the  most  complex 
and  complete  in  its  action  of  the  whole  series.  It  is  amylolytic, 
proteolytic,  and  steaptic,  and  these  powers  have  been  attributed 
to  three  distinct  ferments— amy lopsin,   tripsin,  and  steapsin. 

Proteid  digestion  is  carried  further  than  by  the  gastric  juice, 
and  the  quantity  of  crystalline  nitrogenous  products  formed  is 
in  inverse  proportion  to  the  amount  of  peptone,  from  which  it 
seems  just  to  infer  that  part  of  the  original  peptone  has  been 
converted  into  these  bodies,  which  are  found  to  be  abundant  or 
not  in  an  artificial  digestion,  according  to  the  length  of  time 
it  has  lasted— the  longer  it  has  been  under  way  the  more  leucin 
and  tyrosin  present.  Leucin  is  another  compound  into  which 
the  amido  (NH2)  group  enters  to  make  amido-caproic  acid— one 
of  the  fatty  series— while  tyrosin  is  a  very  complex  member  of 
the  aromatic  series  of  compounds.  Thus  complicated  are  the 
chemical  effects  of  the  digestive  juices  ;  and  it  seems  highly 
probable  that  these  are  only  some  of  the  compounds  into  which 
the  proteid  is  broken  up.     Though  putrefactive  changes  with 


DIGESTION   OF   FOOD. 


307 


formation  of  indol,  etc.,  occur  in  pancreatic  digestion,  both 
within  and  without  the  body,  they  are  to  be  regarded  as  acci- 
dental, for  by  proper  precautions  digestion  may  be  carried  on 


et%*> 


3Ml 


«SB!ao ' 


1  If '^ 


ofi,   I 


Fig.  849.— Micro-organisms  of  large  intestine  (after  Landois),  1.  bacterium  coli  com- 
mune; 2,  bacterium  lactis  aerogenes;  3,  4,  large  bacilli  of  Bienstock,  with  partial 
endogenous  spore-formation;  5,  various  stages  of  development  of  bacillus  which 
causes  fermentation  of  albumen. 


in  the  laboratory  without  their  occurrence,  and  they  vary  in 
degree  with  the  animal,  the  individual,  the  food,  and  other  con- 
ditions. It  is  not,  however,  to  be  inferred  that  micro-organisms 
serve  no  useful  purpose  in  the  alimentary  canal  ;  the  subject, 
in  fact,  requires  further  investigation. 

Succus  Entericus.— The  difficulties  of  collecting  the  secre- 
tions of  Lieberkuhn's,  Briinner's,  and  other  intestinal  glands  will 
be  at  once  apparent.  But  by  dividing  the  intestine  in  two 
places,  so  as  to  isolate  a  loop  of  the  gut,  joining  the  sundered 


Fig.  250.— Portion  of  one  of  Briinner's  glands  (Chauveau). 

ends  by  ligatures,  thus  making  the  continuity  of  the  main  gut 
as  complete  as  before,  closing  one  end  of  the  isolated  loop,  and 


308 


COMPARATIVE  PHYSIOLOGY. 


bringing  the  other  to  the  exterior,  as  a  fistulous  opening,  the 
secretions  could  be  collected,  food  introduced,  etc. 

But  it  seems  highly  improbable  that  information  approxi- 
mately correct  at  best,  and  possibly  highly  misleading,  could 


"m 


•i)-,y) 


Pig.  251.— Intestinal  tubules  (follicles  of  Lieberkiihn)  1  x  100  (Sappey).    A,  from  dog; 
B,  ox;  C,  sheep;  D,  pig;  E,  rabbit. 

be  obtained  in  such  manner.  Moreover,  the  greatest  diversity 
of  opinion  prevails  as  to  the  facts  themselves,  so  that  it  seems 
scarcely  worth  while  to  state  the  contradictory  conclusions  ar- 
rived at. 

It  is,  however,  on  the  face  of  it,  probable  that  the  intestine — 
even  the  large  intestine — does  secrete  juices  that  in  herbivora, 
at  all  events,  play  no  unimportant  part  in  the  digestion  of  their 


Fig.  252. — General  view  of  horse's  intestines;  animal  is  placed  on  its  back,  and  intes- 
tinal mass  spread  out  (after  Ohaiiveau).  A,  duodenum  as  it  passes  behind  great 
mesenteric  artery;  B,  free  portion  of  small  intestine;  0,  ileocaecal  portion;  D, 
creciim;  E,  I<\  G,  loop  formed  by  large  colon;  G,  pelvic  flexure;  F,  F,  point 
where  colic  loop  is  doubled  to  constitute  suprasternal  and  diaphragmatic  flexures. 


Fig.  352. 


310  COMPARATIVE  PHYSIOLOGY. 

bulky  food  ;  and  it  is  also  probable,  as  in  so  many  otber  in- 
stances, that,  when  the  other  parts  of  the  digestive  tract  fail 
when  the  usual  secretions  are  not  prepared  or  do  not  act  on  the 
food,  glands  that  normally  play  a  possibly  insignificant  part 
may  function  excessively — we  may  almost  say  vicariously — 
and  that  such  glands  must  be  sought  in  the  email  intestine. 
There  are  facts  in  clinical  medicine  that  seem  to  point  strongly 
in  this  direction,  though  the  subject  has  not  yet  been  reduced  to 
scientific  form. 

Comparative.— Within  the  last  few  years  the  study  of  vege- 
table assimilation  from  the  comparative  aspect  has  been  fruit- 
ful in  results  which,  together  with  many  other  facts  of  vegeta- 
ble metabolism,  show  that  even  plants  ranking  high  in  the 
organic  plane  are  not  in  many  of  their  functions  so  different 
from  animals  as  has  been  supposed.  It  has  been  known  for  a 
longer  period  that  certain  plants  are  carnivorous  ;  but  it  was 
somewhat  of  a  surprise  to  find,  as  has  been  done  within  the 
past  few  years,  that  digestive  ferments  are  widely  distributed 
in  the  vegetable  kingdom  and  are  found  in  many  different  parts 
of  plants.  What  purpose  they  may  serve  in  the  vegetable 
economy  is  as  yet  not  well  known.  At  present  it  would  seem 
as  though,  from  their  presence  in  so  many  cases  in  the  seed, 
they  might  have  something  to  do  with  changing  the  cruder 
forms  of  nutriment  into  such  as  are  better  adapted  for  the  nour- 
ishment of  the  embryo. 

Thus  far,  then,  not  only  diastase  but  pepsin,  a  body  with 
action  similar  to  trypsin,  and  a  rennet  ferment,  rank  among  the 
vegetable  ferments  best  known. 

A  ferment  has  been  extracted  from  the  stem,  leaves,  and  un- 
ripe fruit  of  Carica  papaya,  found  in  the  East  and  West  Indies 
and  elsewhere,  which  has  a  marked  proteolytic  action. 

It  is  effective  in  a  neutoal,  most  so  in  an  alkaline  medium  ; 
and,  though  its  action  is  suspended  in  a  feeble  acid  menstruum, 
it  does  not  appear  to  be  destroyed  under  such  circumstances,  as 
is  trypsin.  This  body  is  attracting  a  good  deal  of  attention, 
and  its  use  has  been  recently  introduced  into  medical  prac- 
tice. 

Very  lately  also  a  vegetable  rennet  has  been  found  in  sev- 
eral species  of  plants.  The  subject  is  highly  promising  and 
suggestive. 


DIGESTION   OF   FOOD. 


311 


SECRETION   AS   A  PHYSIOLOGICAL   PROCESS. 

Secretion  of  the  Salivary  Glands.— We  shall  treat  this  subject 
at  more  length  because  of  the  light  it  throws  on  the  nervous 
phenomena  of  vital  process  ;  and,  since  the  salivary  glands  have 
been  studied  more  thoroughly  and  successfully  than  any  other, 
they  will  receive  greater  attention. 


Fig.  253. 


Fig.  254. 


Fig.  253.— Lobule  of  parotid  gland,  injected  with  mercury,  and  magnified  50  diame- 
ters. 
Fig.  254.— Capillary  network  around  the  follicles  of  the  parotid  gland. 

The  main  facts,  ascertained  experimentally  and  otherwise, 
are  the  following  : 

Assuming  that  the  student  is  familiar  with  the  general  ana- 
tomical relations  of  the  salivary  glands  in  some  mammal,  we 
would  further  remind  him  that  the  submaxillary  gland  has  a 
double  nervous  supply  :  1.  From  the  cervical  sympathetic  by 
branches  passing  to  tbe  gland  along  its  arteries.  2.  From  the 
chorda  tympani  nerve,  which  after  leaving  the  facial  makes 
connection  with  the  lingual,  whence  it  proceeds  to  its  desti- 
nation. 

The  following  facts  are  of  importance  as  a  basis  for  conclu- 
sions: 1.  It  is  a  matter  of  common  observation  that  a  flow  of 
saliva  may  be  excited  by  the  smell,  taste,  sight,  or  even  thought 
of  food.  2.  It  is  also  a  matter  of  experience  that  emotions,  as 
fear,  anxiety,  etc.,  may  parch  the  mouth — i.  e.,  ari'est  the  flow 
of  saliva.  The  excited  speaker  thus  suffers  in  his  early  efforts. 
3.  If  a  glass  tube  be  placed  in  the  duct  of  the  gland  and  any 
substance  that  naturally  causes  a  flow  of  saliva  be  placed  on 
the  tongue,  saliva  may  be  seen  to  rise  rapidly  in  the  tube.  4. 
The  same  may  be  observed  if  the  lingual  nerve,  the  glossopha- 


312 


COMPARATIVE  PHYSIOLOGY. 


ryngeal,  and  many  other  nerves  be  stimulated ;  also  if  food  be 
introduced  into  the  stomach  through  a  fistula.     5.  If  the  pe- 


Fig.  255.— Maxillary  and  sublingual  gland  (Chauveau).    R,  maxillary  gland;  S,  Whar- 
ton's duct;  T,  sublingual  gland. 

ripheral  end  of  the  chorda  tympani  be  stimulated,  two  results 
follow  :  (a)  There  is  an  abundant  flow  of  saliva,  and  (6)  the 
arterioles  of  the  gland  become  dilated  ;  the  blood  may  pass 
through  with  such  rapidity  that  the  venous  blood  may  be 
bright  red  in  color  and  there  may  be  a  venous  pulse.  7.  Stimu- 
lation of  the  medulla  oblongata  gives  rise  to  a  flow  of  saliva, 
which  is  not  possible  when  the  nerves  of  the  gland,  especially 
the  chorda  tympani,  are  divided;  nor  can  a  flow  be  then  excited 
by  any  sort  of  nervous  stimulation,  excepting  that  of  the  ter- 
minal branches  of  the  nerves  of  the  gland  itself.  8.  If  the  sym- 
pathetic nerves  of  the  gland  be  divided,  there  is  no  immediate 
flow  of  saliva,  though  there  may  be  some  dilatation  of  its  ves- 


DIGESTION  OF  FOOD. 


313 


sels.  9.  Stimulation  of  the  terminal  ends  of  the  sympathetic 
and  chorda  nerves  causes  a  flow  of  saliva,  differing  as  to  total 
quantity  and  the  amount  of  contained  solids;  hut  the  nerve 
that  produces  the  more  abundant  watery  secretion,  or  the  re- 
verse, varies  with  the  animal,  e.  g.,  in  the  cat  chorda  saliva  is 


Part  of  brain  above  medulla 


Afferent  nerves 
from  tongue- 


Fig.  256.  -Diagram  intended  to  indicate  the  nervous  mechanism  of  salivary  secretion. 


more  viscid,  in  the  dog  less  so ;  though  in  all  animals  as  yet 
examined  it  seems  that  the  secretion  as  a  result  of  stimulation 
of  the  chorda  tympani  nerve  is  the  most  abundant ;  and  in  the 


314  COMPARATIVE  PHYSIOLOGY. 

case  of  stimulation  of  the  chorda  the  vessels  of  the  gland  are 
dilated,  while  in  the  case  of  the  sympathetic  they  are  con- 
stricted. 10.  If  atropin  be  injected  into  the  blood,  it  is  impos- 
sible to  induce  salivary  secretion  by  any  form  of  stimulation, 
though  excitation  of  the  chorda  nerve  still  causes  arterial  dila- 
tation. 

Conclusions. — 1.  There  is  a  center  in  the  medulla  presiding 
over  salivary  secretion.  2.  The  influence  of  this  center  is  ren- 
dered effective  through  the  chorda  tympani  nerve  at  all  events, 
if  not  also  by  the  sympathetic.  3.  The  chorda  tympani  nerve 
contains  both  secretory  and  vaso-dilator  fibers;  the  sympathetic 
secretory  and  vaso-constrictor  fibers.  4.  Arterial  change  is  not 
essential  to  secretion,  though  doubtless  it  usually  accompanies 
it.  Secretion  may  be  induced  in  the  glands  of  an  animal  after 
decapitation  by  stimulation  of  its  chorda  tympani  nerve,  analo- 
gous to  the  secretion  of  sweat  in  the  foot  of  a  recently  dead 
animal,  under  stimulation  of  the  sciatic  nerve.  5.  The  char- 
acter of  the  saliva  secreted  varies  with  the  nerve  stimulated,  so 
that  it  seems  likely  that  the  nervous  centers  normally  in  the 
intact  animal  regulate  the  quality  of  the  saliva  through  the 
degree  to  which  one  or  the  other  kind  of  nerves  is  called  into 
action.  6.  Secretion  of  saliva  may  be  induced  reflexly  by  ex- 
periment, and  such  is  probably  the  normal  course  of  events. 
7.  The  action  of  the  medullary  center  may  be  inhibited  by  the 
cerebrum  (emotions). 

Some  have  located  a  center  in  the  cerebral  cortex  (taste  cen- 
ter), to  which  it  is  assumed  impulses  first  travel  from  the 
tongue  and  which  then  rouses  the  proper  secreting  centers  in 
the  medulla  into  activity.  It  seems  more  likely  that  the  corti- 
cal center,  if  there  be  one,  completes  the  physiological  processes 
by  which  taste  sensations  are  elaborated. 

From  the  influence  of  drugs  (atropin  and  its  antagonist 
pilocarpin)  it  is  plain  that  the  gland  can  be  effected  through 
the  blood,  though  whether  wholly  by  direct  action  on  the  cen- 
ter, on  any  local  nervous  mechanism  or  directly,  on  the  cells,  is 
as  yet  undetermined.  It  is  found  that  pilocarpin  can  act  long 
after  section  of  the  nerves.  This  does  not,  however,  prove  that 
in  the  intact  animal  such  is  the  usual  modus  operandi  of  this 
or  other  drugs,  any  more  than  the  so-called  paralytic  secretion 
after  the  section  of  nerves  proves  that  the  latter  are  not  con- 
cerned in  secretion. 

We  look  upon  paralytic  secretion  as  the  work  of  the  cells 


DIGESTION   OP   FOOD.  315 

when  gone  wrong — passed  from  under  the  dominion  of  the 
nerve-centers.  Secretion  is  a  part  of  the  natural  life-processes 
of  gland-cells — we  may  say  a  series  in  the  long  chain  of  pro- 
cesses which  are  indispensable  for  the  health  of  these  cells. 
They  must  be  either  secreting  cells,  or  have  no  place  in  the  nat- 
ural order  of  things.  It  is  to  be  especially  noted  that  the  secre- 
tion of  saliva  continues  when  the  pressure  in  the  ducts  of  the 
gland  is  greater  than  that  of  the  blood  in  its  vessels  or  even 
of  the  carotid ;  so  that  it  seems  possible  that  over-importance 
has  been  attached  to  blood-pressure  in  secretory  processes  gen- 
erally. 

It  may,  then,  be  safely  assumed  that  formation  of  saliva  re- 
sults in  consequence  of  the  natural  activity  of  certain  cells,  the 
processes  of  which  are  correlated  and  harmonized  by  the  nerv- 
ous system ;  their  activity  being  accompanied  by  an  abundant 
supply  of  blood.  The  actual  outpouring  of  saliva  depends  usu- 
ally on  the  establishment  of  a  nervous  reflex  arc.  The  other 
glands  have  been  less  carefully  studied,  but  the  parotid  is 
known  to  have  a  double  nervous  supply  from  the  cerebro- 
spinal and  the  sympathetic  systems. 

It  would  appear  that,  as  the  vaso-motor  changes  run  paral- 
lel with  the  secretory  ones,  the  vaso-motor  and  the  proper 
secretory  centers  act  in  concert,  as  we  have,  seen  holds  of  the 
former  and  the  respiratory  center.  But  it  is  to  our  own  mind 
very  doubtful  whether  the  doctrine  of  so  sharp  a  demarkation 
of  independent  centers,  prominently  recognized  in  the  physi- 
ology of  the  day,  will  be  that  ultimately  accepted. 

Secretion  by  the  Stomach.— The  mucous  membrane  of  St. 
Martin's  stomach  was  observed  (through  an  accidental  fistulous 
opening)  to  be  pale  in  the  intervals  of  digestion,  but  flushed 
when  secreting,  which  resembled  sweating,  so  far  as  the  flow 
of  the  fluid  is  concerned.  When  the  man  was  irritated,  the 
gastric  membrane  became  pale,  and  secretion  was  lessened  or 
arrested,  and  it  is  a  common  experience  that  emotions  may 
help,  hinder,  or  even  render  aberrant  the  digestive  processes. 

While  the  evidence  is  thus  clear  that  gastric  secretion  is 
regulated  by  the  nervous  system,  the  way  in  which  this  is  ac- 
complished is  very  obscure.  We  know  little  of  either  the  cen- 
ters or  nerves  concerned,  and  what  we  do  know  helps  but 
doubtfully  to  an  understanding  of  the  matter,  if,  indeed,  it  does 
not  actually  confuse  and  puzzle. 

Digestion  can  proceed  in  a  fashion  after  section  of  the  nerves 


316 


COMPARATIVE   PHYSIOLOGY. 


going  to  the  stomach,  though  this  has  little  force  as  an  argu- 
ment against  nerve  influence.  We  may  conclude  the  subject 
by  stating  that,  while  the  influence  of  the  nervous  system  over 
gastric  secretion  is  undoubted  as  a  fact,  the  method  is  not  un- 
derstood ;  and  the  same  remark  applies  to  the  secreting  activity 
of  the  liver  and  pancreas. 

The  Secretion  of  Bile  and  Pancreatic  Juice. — When  the  con- 
tents of  the  stomach  have  reached  the  orifice  of  the  discharging 
bile-duct,  a  large  flow  of  the  biliary  secretion  takes  place,  prob- 
ably as  the  result  of  the  emptying  of  the  gall-bladder  by  the 
contraction  of  its  walls  and  those  of  its  ducts.  This  is  probably 
a  reflex  act,  and  the  augmented  flow  of  bile  when  digestion  is 
proceeding  is  also  to  be  traced  chiefly  to  nervous  influences 
reaching  the  gland,  though  by  what  nerves  or  under  the  gov- 
ernment of  what  part  of  the  nervous  centers  is  unknown. 
Very  similar  statements  apply  to  the  secretion  of  the  pancre- 


Fig.  257.— Diagram  to  show  influence  of  food  in  secretion  of  pancreatic  juice  (after 
N.  O.  Bernstein).     The  abscissa?  represent  hours  after  taking  food  ;  ordinates 
amount  in  cubic  centigrammes  of  secretion  in  ten  minutes.     Food  was  taken  at 
•  Ji  und  C.    This  diagram  very  nearly  also  represents  the  secretion  of  bile. 


atic  glands,  though  this  is  not  constant,  as  in  the  case  of  bile — 
at  all  events  in  most  animals. 

It  is  known  that  after  food  has  been  taken  there  is  a  sudden 


DIGESTION   OP   POOD. 


317 


increase  in  the  quantity  of  bile  secreted,  followed  by  a  sudden 
diminution,  then  a  more  gradual  rise,  with  a  subsequent  fall. 
Almost  the  same  holds  for  the  pancreas. 

It  seems  impossible  to  explain  tbese  facts,  especially  the 
first  rapid  discharge  of  fluid  apart  from  the  direct  influence  of 
the  nervous  system. 

Upon  the  whole,  the  evidence  seems  to  show  that  the  press- 
ure in  the  bile-ducts  is  greater  than  in  the  veins  that  unite  to 
make  up  the  portal  system;  but  there  are  difficulties  in  the 
investigation  of  such  and  kindred  subjects  as  regards  the  liver, 
owing  to  its  peculiar  vascular  supply.  It  will  be  borne  in  miud 
that  the  liver  in  mammals  consists  of  a  mass  of  blood-vessels, 
between  the  meshes  of  which  are  packed  innumerable  cells,  and 
that  around  the  latter  meander  the  bile  capillaries;  that  the 
portal  vein  breaks  up  into  the  intralobular,  from  which  capil- 
laries arise,  that  terminate  in  the  central  interlobular  veins, 
which  make  up  the  hepatic  veinlets  or  terminate  in  these  vessels. 
But  the  structure  is  complicated  by  the  branches  of  the  hepatic 
artery,  which,  as  arterioles  and  capillaries,  enters  to  some  extent 
into  the  formation  of  the  lobular  vessels. 

A  question  of  interest,  though  difficult  to  answer,  is  the 
extent  to  which  the  various  constituents  of  bile  are  manufact- 
ured in  the  liver.     Taurin,  for  example,  is  present  in  some  of 


-  wo        \-.  rT-      "Mil  "-TT    *f    .-•  '". 


Pig.  258.— Lobules  of  liver,  interlobular  vessels,  and  intralobular  veins  (Sappey).  1, 1, 
1, 1,  3,  4,  lobules;  2,2,  2,  2,  intralobular  veins  injected  with  white;  5,  5,  5,  5,5,  in- 
tralobular vessels  tilled  with  a  dark  injection. 


318 


COMPARATIVE   PHYSIOLOGY. 


the  tissues,  but  whether  this  is  used  in  the  manufacture  of 
taurocholic  acid  or  whether  the  latter  is  made  entirely  anew, 
and  possibly  by  a  method  in  which  taurin  never  appears  as 
such,  is  an  open  question.  It  is  highly  probable  that  a  portion 
of  the  bile  poured  into  the  intestine  is  absorbed  either  as  such 
or  after  partial   decomposition,   the   products   to  be  used   in 

some  way  in  the  econo- 
§|||  ';vA  my  and  presumably  in 
the  construction  of  bile 
by  the  liver.  There  are 
many  facts,  including 
some  pathological  phe- 
nomena, that  point 
clearly  to  the  formation 
of  the  pigments  of  bile 
from  haemoglobin  in 
some  of  its  stages  of  de- 
generation. 

Pathological.— When 
the  liver  fails  to  act, 
either  from  derange- 
ment of  its  cells  prima- 
rily or  owing  to  obstruc- 
tion to  the  outflow  of 
bile  leading  to  reabsorp- 
tion  by  the  liver,  bile  acids  and  bile  pigments  appear  in  the 
urine  or  may  stain  the  tissues,  indicating  their  presence  in  ex- 
cess in  the  blood. 

This  action  of  one  gland  (kidneys)  for  another  is  highly 
suggestive,  and  especially  important  to  bear  in  mind  in  medical 
practice,  both  in  treatment  and  prognosis.  The  chances  of  re- 
covery when  only  one  excreting  gland  is  diseased  are  much 
greater  evidently  than  when  several  are  involved.  Such  facts 
as  we  have  cited  show,  moreover,  that  there  are  certain  common 
fundamental  principles  underlying  secretion  everywhere— a 
statement  which  will  be  soon  mbre  fully  illustrated. 


Fig.  259.  —Portion  of  transverse  section  of  hepatic 
lobule  of  rabbit  magnified  400  diameters  (K51H- 
ker).  b,\b,  b,  capillary  blood-vessels;  g,  g,  g,  cap- 
■    illary  bile-ducts;  I,  1,1,  liver-cells. 


THE  NATURE  OF  THE  ACT  OF  SECRETION. 

We  are  now  about  to  consider  some  investigations,  more 
particularly  their  results,  which  are  of  extraordinary  interest. 
The  secreting  cells  of  the  salivary,  the  pancreatic  glands, 


DIGESTION   OF   FOOD. 


310 


and  the  stomach  have  been  studied  by  a  combination  of  histo- 
logical and,  more  strictly,  physiological  methods,  to  which  we 
shall  now  refer.  Specimens  of  these  glands,  both  before  and 
after  prolonged  secretion,  under  stimulation  of  these  nerves, 


Fig.  260.— Portion  of  pancreas  of  rabbit  (after  Kiihne  and  Lea), 
at  rest;  B,  during  secretion. 


A  represents  gland 


were  hardened,  stained,  and  sections  prepared.  As  was  to  be 
expected,  the  results  were  not  entirely  satisfactory  under  these 
methods  ;  however,  the  pancreas  of  a  living  rabbit  has  been 
viewed  with  the  microscope  in  its  natural  condition  ;  and  by 
this  plan,  especially  when  supplemented  by  the  more  involved 
and  artificial  method  first  referred  to,  results  have  been  reached 
which  may  be  ranked  among  the  greatest  triumphs  of  modern 
physiology. 

Some  of  these  we  now  proceed  to  state  briefly.  To  begin 
with  the  pancreas,  it  has  been  shown  that,  when  the  gland  is 
not  secreting — i.  e.,  not  discharging  its  prepared  fluid — or  dur- 
ing the  so-called  resting  stage,  the  appearances  are  strikingly 
different  from  what  they  are  during  activity.  The  cell  pre- 
sents during  rest  an  inner  granular  zone  and  an  outer  clearer 
zone,  which  stains  more  readily,  and  is  relatively  small  in  size. 
The  lumen  of  the  alveolus  is  almost  obliterated,  and  the  in- 
dividual cells  very  indistinct.  After  a  period  of  secreting 
activity,  the  lumen  is  easily  perceived,  the  granules  have  dis- 
appeared in  great  part,  the  cells  as  a  whole  are  smaller,  and 
have  a  clear  appearance  throughout.  Coincident  with  the 
changes  in  the  gland's  cells  it  is  to  be  noticed  that  more  blood 
passes  through  it,  owing  to  dilatation  of  the  arterioles. 

Again,  the  course  of  the  changes  in  the  salivary  glands, 
whether  of  the  mucous  or  serous  variety,  is  very  similar.     In 


320 


COMPARATIVE   PHYSIOLOGY. 


the  mucous  gland  in  the  resting  stage  the  cells  are  large,  and 
hold  much  clear  matter  in  the  interspaces  of  the  cell  network ; 


Pig.  261. — Section  of  mucous  gland  (after  Lavdowsky).    In  A,  gland  at  rest;  in  B, 
after  secreting  for  some  time. 

and,  as  this  does  not  stain  readily,  it  can  not  be  ordinary 
protoplasm.  This,  when  the  gland  is  stimulated  through  its 
nerves,  disappears,  leaving  the  containing  cells  smaller.  It 
has  become  mucin,  and  may  itself  be  called  mucinogen. 

It  is  to  be  noted  that,  as  the  cells  become  more  protoplasmic, 
less  burdened  with  the  products  of  their  activity,  the  nucleus 
becomes  more  prominent,  suggestive  of  its  having  a  probable 
directive  influence  over  these  manufacturing  processes. 

Substantially  the  same  chain  of  events  has  been  established 
for  the  serous  salivary  glands  and  the  stomach,  so  that  we 
may  safely  generalize  upon  these  well-established  facts. 

It  seems  clear  that  a  series  of  changes  constructive  and,  from 
one  point  of  view,  destructive,   following  the  former  are  con- 


Fig.  202.— ( 'hanges  in  parotid  (serous)  gland  during  secretion  (after  Langley).  A,  dur- 
ing rest;  B,  after  moderate,  C,  after  prolonged  stimulation.  Figures  partly  dia- 
grammatic. 

stantly  going  on  in  the  glands  of  the  digestive  organs.     Proto- 
plasm under  nerve  influence  constructs   a  certain  substance, 


DIGESTION   OF  FOOD.  321 

which  is  an  antecedent  of  the  final  product,  which  we  term  a 
ferment.  It  is  now  customary  to  speak  of  these  changes  as 
constructive  (anabolic)  and  destructive  (katabolic),  though  we 
have  already  pointed  out  (page  258)  that  this  view  is,  at  best, 
only  one  way  of  looking  at  the  matter,  and  we  doubt  if  it  may 
not  be  cramping  and  misleading. 

We  must  also  urge  caution  in  regard  to  the  conception  to 
be  associated  with  the  use  of  the  terms  "  resting  "  and  "  active  " 
stage.  It  is  not  to  be  forgotten  that  strictly  in  living  cells 
there  is  no  absolute  rest — such  means  death  ;  but,  if  these  terms 
be  understood  as  denoting  but  degrees  of  activity,  they  need 
not  mislead.  It  is  also  more  than  probable  that  in  certain  of 
the  glands,  or  in  some  animals,  the  processes  go  on  simultane- 
ously ;  the  protoplasm  being  renewed,  the  zymogen,  or  mother- 
ferment,  being  formed,  and  the  latter  converted  into  actual  fer- 
ment, all  at  the  same  time. 

The  nature  of  secretion  is  now  tolerably  clear  as  a  wbole ; 
though  it  is  to  be  remembered  that  this  account  is  but  general, 
and  that  there  are  many  minor  differences  for  each  gland  and 
variations  that  can  scarcely  be  denominated  minor  for  different 
animals.  Evidently  no  theory  of  filtration,  no  process  depend- 
ing solely  on  blood-pressure,  will  apply  here.  And  if  in  this, 
the  best-studied  case,  mechanical  theories  of  vital  processes 
utterly  fail,  why  attempt  to  fasten  them  upon  other  glands,  as 
the  kidneys  and  the  lungs,  or,  indeed,  apply  such  crude  concep- 
tions to  the  subtle  processes  of  living  protoplasm  anywhere  or 
in  any  form  \ 

It  is  somewhat  remarkable  that  an  extract  of  a  perfectly 
fresh  pancreas  is  not  proteolytic ;  yet  the  gland  yields  such  an 
extract  when  it  has  stood  some  hours  or  been  treated  with  a 
weak  acid.  These  facts,  together  with  the  microscopic  appear- 
ances, suggested  that  there  is  formed  a  forerunner  to  the  actual 
ferment — a  zymogen,  or  mother-ferment,  which  at  the  moment 
of  discharge  of  the  completed  secretion  is  converted  into  the 
actual  ferment.  We  might,  therefore,  speak  of  a  pepsinogen, 
trypsinogen,  etc.,  and,  though  there  may  be  a  cessation  in  the 
series  of  processes,  and  no  doubt  there  is  in  some  animals,  this 
may  not  be  the  case  in  all,  or  in  all  glands. 

Secretion  by  the  Stomach.— The  glands  of  the  stomach  differ 

in  most  animals  in  the  cardiac  and  pyloric  regions.     In  those 

of  the  former  zone,  both  central  (columnar)  and  parietal  (ovoid) 

cells  are  to  be  recognized.     It  was  thought  that  possibly  the  lat- 

21 


#s    Mm 


-ft1.*) 

.vt;«l 


5;. 

"1 


Fig.  263. 


Pig.  364. 


Fio.  265. 


Pig,  268.—  Piii-  m  mucous  membrane  of  stomach  in  which  arc  openings  of  tubular 

glands,  1  x  20  (Sappey). 
Pig.  364.-  Glands  of  stomach  with  both  central  and  parietal  (ovoid)  cells  (Heidenhain). 
Fi<;.  865.-  -  Pyloric,  glands  (Ebstein). 


DIGESTION   OF   FOOD.  323 

ter  were  concerned  in  the  secretion  of  the  acid  of  the  stomach, 
hut  this  is  by  no  means  certain.  Possibly  these,  like  the  demi- 
lune cells  of  the  pancreas,  may  be  the  progenitors  of  the  central 
(chief)  cells.  The  latter  certainly  secrete  pepsin,  and  probably 
also  rennet.  Mucus  is  secreted  by  the  cells  lining  the  neck  of 
glands  and  covering  the  mucous  membrane  intervening  be- 
tween their  mouths.  The  production  of  hydrochloric  acid  by 
any  act  of  secretion  is  not  believed  in  by  all  writers,  some  hold- 
ing that  it  is  derived  from  decomposition  of  sodium  chloride, 
possibly  by  lactic  acid.  So  simple  an  origin  is  not  probable,  not 
being  in  keeping  with  what  we  know  of  chemical  processes 
within  the  animal  body. 

Self-Digestion  of  the  Digestive  Organs.— It  has  been  found, 
both  in  man  and  other  mammals,  that  when  death  follows  in  a 
healthy  subject  while  gastric  digestion  is  in  active  progress 
and  the  body  is  kept  warm,  a  part  of  the  stomach  itself  and 
often  adjacent  organs  are  digested,  and  the  question  is  con- 
stantly being  raised,  Why  does  not  the  stomach  digest  itself 
during  life  ?  To  this  it  has  been  answered  that  the  gastric 
juice  is  constantly  being  neutralized  by  the  alkaline  blood  ; 
and,  again,  that  the  very  vitality  of  a  tissue  gives  it  the  neces- 
sary resisting  powers,  a  view  contradicted  by  an  experiment 
which  is  conclusive.  If  the  legs  of  a  living  frog  be  allowed 
to  hang  against  the  inner  walls  of  the  stomach  of  a  mammal 
when  gastric  digestion  is  going  on,  they  will  be  digested. 

The  first  view  (the  alkalinity  of  the  blood)  would  not  suffice 
to  explain  why  the  pancreas,  the  secretion  of  which  acts  best  in 
an  alkaline  medium,  should  not  be  digested. 

It  seems  to  us  there  is  a  good  deal  of  misconception  about 
the  facts  of  the  case.  Observation  on  St.  Martin  shows  that 
the  secretion  of  gastric  juice  runs  parallel  with  the  need  of  it, 
as  dependent  on  the  introduction  of  food,  its  quantity,  quality, 
etc.  Now,  there  can  be  little  doubt  that,  if  the  stomach  were 
abundantly  bathed  when  empty  with  a  large  quantity  of  its 
own  acid  secretion,  it  would  suffer  to  some  extent  at  least.  But 
this  is  never  the  case  ;  the  juice  is  carried  off  and  mixed  with 
the  food.  This  food  is  in  constant  motion  and  doubtless  the 
inner  portions  of  the  cells,  which  may  be  regarded  as  the  dis- 
charging region  (the  outer,  next  tbe  blood  capillaries,  being 
the  chief  manufacturing  region  of  the  digestive  ferment),  are 
frequently  renewed. 

Such  considerations,  though  they  seem  to  have  been  some- 


324 


COMPARATIVE   PHYSIOLOGY. 


what  left  out  of  the  case,  do  not  go  to  the  bottom  of  the  matter. 
Amoeba  and  kindred  organisms  do  not  digest  themselves. 
Some  believe  that  the  little  pulsatile  vacuoles  of  the  Infusorians 
are  a  sort  of  temporary  digestive  cavities. 

But,  to  one  who  sees  in  the  light  of  evolution,  it  must  be 
clear  that  a  structure  could  not  have  been  evolved  that  would 
be  self -destructive. 

The  difficulty  here  is  that  which  lies  at  the  very  basis  of  all 
life.  We  might  ask,  Why  do  living  things  live,  since  they  are 
constantly  threatened  with  destruction  from  within  as  from 
without  ?  Why  do  not  the  liver,  kidney,  and  other  glands  that 
secrete  noxious  substances,  poison  themselves  ?  We  can  not 
in  detail  explain  these  things  ;  but  we  wish  to  make  it  clear 
that  the  difficulty  as  regards  the  stomach  is  not  peculiar  to  that 
gland,  and  that  even  from  the  ordinary  point  of  view  it  has 
been  exaggerated. 

Comparative. — More  careful  examination  of  the  stomachs  of 
some  mammals  has  revealed  the  fact  that  in  several  animals, 

in  which  the  stomach  appears  to 
be  simple,  it  is  in  reality  com- 
pound. There  are  different 
grades,  however,  which  may  be 
regarded  as  transition  forms  be- 
tween the  true  simple  stomach 
and  that  highly  compound  form 
of  the  organ  met  with  in  the 
ruminants. 

It  has  been  shown  recently 
that  the  stomach  of  the  hog  has 
an  oesophageal  dilatation  ;  and 
that  the  entire  organ  may  be 
divided  into  several  zones  with 
different  kinds  of  glandular  epi- 
thelium, etc.  These  portions 
differ  in  digestive  power,  in  the  characteristics  of  the  fluid  se- 
creted, and  other  details  beyond  those  which  a  superficial  exam- 
ination of  this  organ  would  lead  one  to  suspect. 

The  stomach  of  the  horse  represents  a  more  advanced  form 
of  compound  stomach  than  that  of  the  hog,  which  is  not  evi- 
dent, however,  until  its  glandular  structure  is  examined  closely. 
The  entire  left  portion  of  the  stomach  represents  an  oesophageal 
dilatation  lined  with  an  epithelium  that  closely  resembles  that 


Fig.  266.— Interior  of  horse's  stomach 
(after  Chauveau).  A.  left  sac;  B, 
right  sac;   0,  duodenal  dilatation. 


DIGESTION   OF   FOOD. 


325 


of  the  oesophagus,  and  with  little  if  auy  digestive  function.  It 
thus  appears  that  the  stomach  of  the  horse  is  in  reality  smaller, 
as  a  true  digestive  gland,  than  it  seems,  so  that  a  great  part  of 
the  work  of  digestion  must  he  done  in  the  intestine  ;  though  in 
this  animal,  if  the  food  be  retained  as  long  as  it  is  in  the  hog. 
which  is  not,  however,  the  general  opinion  as  regards  the 
stomach  of  the  horse,  salivary  digestion  may  continue  for  a 
considerable  period  after  the  food  has  left  the  mouth.  The 
secretion  of  mucus  by  the  stomach  in  herbivora  is  abundant. 

As  has  been  already  explained,  the  stomach  of  ruminants 
consists  of  several  compartments  which  are  supplementary  to 
one  another,  though  genuine  gastric  digestion  does  not  take 
place  except  in  the  fourth  stomach. 

The  first  and  second  stomachs  being  destitute  of  other  than 
mucous  glands,  and  lined  with  a  horny  epithelium,  are  to  be  con- 
sidered rather  as  dilatations  of  the  oesophagus.  They  answer 
admirably  the  purpose  of  storehouses  for  the  bulky  food  in 
which  the  softening  process  preparatory  to  mastication  goes  on. 


Pig.  267. — Stomach  of  the  ox  scon  on  its  right  upper  face,  tiie  aboniasum  being  de- 
pressed (Chauveau).  A.  rumen,  left  hemisphere;  B,  rumen,  right  hemisphere;  ('. 
termination  of  the  oesophagus;  D,  reticulum;  E.  omasum;  P,  abomasnm. 


326 


COMPARATIVE   PHYSIOLOGY. 


Fig.  268. — Interior  of  stomach  in  ruminants;  the  upper  plane  of  the  rumen  and  reticu- 
lum, with  the  oesophageal  furrow  (Chauveau).  A,  left  sac  of  the  rumen;  B,  ante- 
rior extremity  of  that  sac  turned  back  on  right  sac;  0,  its  posterior  extremity,  or 
left  conical  cyst;  G,  section  of  anterior  pillar  of  rumen;  g,  g,  its  two  superior 
branches;  H,  posterior  pillar  of  same;  h,  h,  h,  its  three  inferior  branches;  I,  cells 
of  reticulum;  J.  oesophageal  furrow;  K,  oesophagus;  L,  abomasum. 


Fig.  269.  Stomach  of  llama  (Colin).  A,  lower  extremity  of  gullet;  B,  single  pillar  ot 
oesophageal  canal:  C,  superior  opening  of  the  psalter;  D,  reticulum;  E,  right  or 
anterior  water-cells;  W,  inferior  water-cells;  G,  fleshy  column  separating  the  two 
cell  groups. 


DIGESTION   OF   FOOD. 


327 


The  reticulum,  so  called  from  the  peculiar  arrangement  of 
the  mucus  membrane,  is  usually  regarded  as  a  receptacle  for 
water  more  especially ;  however,  this  stomach  is  to  be  regarded 
both  anatomically  and  lihysiologically  as  a  subdivision  of  the 
first,  or  at  all  events  as  equivalent  to  that. 

The  quantity  of  food  that  it  can  hold  in  the  ox  is  enormous, 
(150  to  200  pounds),  a  condition  of  things  advantageous  in  an 


Fig.  270. — Omasum  and  abomasum  of  ox  cut  open  (Smith).  A.  psalterium,  with  open- 
ing between  it  and  the  reticulum  at  B;  P.  foldings  (plicse)  of  mucous  membrane 
at  C.  fourth  stomach. 


animal  feeding  upon  substances  so  poor  in  nutritive  material  in 
proportion  to  their  bulk  and  requiring  so  much  mastication  to 
fit  them  to  be  acted  on  by  the  digestive  juices.  The  reaction 
of  tbe  first  two  stomachs  is  alkaline. 

In  the  camel  tribe,  water  cells  are  arranged  in  parallel  order 
in  the  rumen.     The  edges  of  these  are  provided  with  muscular 


328 


COMPARATIVE   PHYSIOLOGY. 


fibers  constituting  sphincters  by  which  their  openings  inward 
may  be  closed.  These  cells  number  several  hundred,  and  are 
capable  of  containing  some  quarts  of  water. 


Pig.  271.— A.  Stomach. of  sheep.  B.  Stomach  of  musk-deer,  m,  oesophagus;  Rn,  ru- 
men; Ret.  reticulum;  Ps,  psaltesium;  A,  Ab,  abomasum;  Bit,  duodenum;  Pij, 
pylorus  (Huxley). 

The  manyplies  is  so  named  from  the  arrangement  of  its 
mucus  membrane  in  folds,  a  condition,  however,  not  equally 
well  marked  in  all  ruminants. 

A  structure  known  as  the  oesophageal  canal,  (furrow,  groove) 
communicates  with  the  first  three  stomachs.  During  swallow- 
ing, its  lower  portion  is  raised  above  the  level  of  the  third 
stomach,  so  that  it  is  likely  that  this  is  a  barrier  against  the 
entrance  of  all  except  liquids  or  soft  foods  into  the  manyplies. 

It  is  difficult  to  make  any  positive  statement  as  to  what  other 
part  it  may  take  in  determining  the  direction  of  food  when  en- 
tering or  leaving  the  various  stomachs.  It  does  not  seem  to  be 
C'ssential  to  return  of  the  cud. 


DIGESTION   OF  FOOD. 


329 


The  abomasum  or  rennet  resembles  other  forms  of  true 
digestive  stomachs  in  all  essential  particulars. 

While  the  opening  between  the  first  and  second  stomachs  is 
large  enough  to  allow  of  free  intercommunication,  the  reverse 
applies  to  the  entrance  into  the  third  stomach. 

The  rumen  is  nearly  always  tolerably  well  filled  with  food,  a 
condition  of  things  favorable  to  its  return  for  remastication. 


Pig.  272. — Stomach  of  horse  (after  Chauveau).    A,  cardiac  extremity  of  oesophagus; 
B,  pyloric  ring. 

We  may  conclude  that  only  food  in  a  proper  form  for  the 
action  of  the  fourth  stomach  passes  to  any  extent  beyond  the 
first  two. 

After  the  food  has  been  duly  softened  and  has  undergone 
some  fermentative  changes  in  the  rumen,  leading  to  the  evolution 
of  gases  (C02,  H2S)  and  certain  organic  acids  (acetic,  butyric), 
it  is  regurgitated  by  a  process  that  closely  resembles  vomiting. 

In  this  the  diaphragm  and  the  abdominal  muscles,  as  well 


330 


COMPARATIVE   PHYSIOLOGY. 


as  the  stomach  itself  and  the  gullet,  take  part.  Probably  as  a 
result  of  the  descent  of  the  diaphragm  and  consequent  diminu- 
tion of  the  intrathoracic  pressure,  the  asceut  of  the  cud  is  as- 
sisted by  an  aspiratory  process.  The  returning  food  is  pre- 
vented from  passing  into  the  nasal  chambers  by  co-ordinated 
movements  analogous  to  those  of  swallowing.  The  whole  pro- 
cess is  reflex  in  the  same  sense  as  is  deglutition. 

Normally  the  rumen  always  contains  considerable  liquid,  a 
portion  of  which  passes  up  with  the  cud,  but  is  in  great  part 
returned  at  once.  A  ruminant  given  dry  food  without  water 
can  not  return  the  cud. 

In  the  second  mastication  the  process  is  in  most  ruminants 
unilateral  ;  and  as  hundreds  of  cuds  are  to  be  chewed,  a  con- 
siderable proportion  of  the  whole  day  is  occupied  with  rumina- 
tion. When  a  single  cud  is  sufficiently  masticated  it  is  swallowed, 


Pig.  273.— Stomach  of  dog  (after  Chaveau).    A,  oesophagus;  B,  pylorus. 


and  being  finely  comminuted  passes  at  once  through  the  small 
opening  between  the  reticulum  and  manyplies  into  the  third 
stomach,  and  thence  into  the  abomasum,  though  possibly  on 
the  way  a  little  may  pass  into  the  first  two  stomachs. 

Pathological. — While  moderate   fullness   of    the  paunch  is 


DIGESTION   OF   FOOD.  331 

favorable  to  rumination,  extreme  distention  tends  to  paralysis 
of  the  muscular  coat  of  the  organ,  allowing-  of  the  accumulation 
of  the  gases  of  fermentation  which  may  lead,  if  not  artificially 
relieved,  to  rupture  of  the  organ. 

THE  ALIMENTARY  CANAL  OF  THE  VERTEBRATE. 

Amid  all  variations  in  this  great  group,  the  alimentary  canal 
has  common  features,  both  of  structure  and  function.  Through- 
out the  entire  tract  muscle  cells  of  the  unstriped  (involuntary) 
kind,  arranged  in  two  layers,  constitute  the  motor  mechanism 
for  the  transportation  of  food  from  one  part  to  another.  Out- 
side of  these  is  the  serous  coat,  consisting  of  fibrous  and  elastic 
tissue,  and  admirably  adapted  to  preserve  organs  from  undue 
distention,  at  the  same  time  providing  a  smooth  external  cover- 
ing which  lessens  the  friction  of  one  organ  against  another  in 
the  abdominal  cavity  ;  while  folds  of  such  tissue  constitute  the 
omentum  for  supporting  the  various  organs. 

Between  the  muscular  and  mucous  coats  of  the  organs  that 
constitute  the  alimentary  canal  there  is  a  submucous  coat  of 
loose  connective  tissue  in  which  ramify  blood-vessels,  nerves,  etc. 

It  is  the  mucous  coat,  however,  that  is  of  paramount  impor- 
tance, and  for  which  all  other  parts  may  in  some  sense  be  con- 
sidered to  exist  ;  for  it  is  from  the  glands  with  which  it  is  sup- 
plied that  the  digestive  juices  are  derived,  as  well  as  that  mucus 
which  keeps  the  tract  moist  and  its  delicate  structures  shielded 
under  all  circumstances.  The  amount  of  surface  provided  by 
the  mucous  membrane  is  increased  by  its  various  foldings 
(rugce,  valvidce  conniventes,  etc.),  so  generally  present,  and 
which  also  allow  of  distention  ;  and  if  the  secreting  glands 
are  regarded  as  minute  induplications  of  this  coat,  it  will 
be  evident  that  its  total  area  is  much  greater  than  at  first  ap- 
pears. 

While  each  part  has  glands  with  structure  peculiar  to  them- 
selves, it  may  be  noticed  that  all  the  essential  epithelium  has  a 
tendency  to  assume  a  somewhat  cubical  form. 

The  secreting  glands  of  the  stomach  and  intestines  are  tubu- 
lar ;  while  the  salivary  glands,  the  pancreas,  and  the  liver  are 
masses  of  cells  so  packed  together  as  to  form  great  colonies  of 
cells  with  lesser  subdivisions  (lobules),  the  whole  being  bound 
together  by  some  form  of  connective  tissue,  and  well  supplied 
with  blood-vessels  and  nerves,  thus  constituting  organs  with  a 


332  COMPARATIVE   PHYSIOLOGY. 

covering  (capsule)  in  structure  allied  to  the  serous  covering  of 
the  stomach  and  intestines. 

Details  will  be  referred  to  in  various  parts  of  the  sections 
devoted  to  this  subject  as  far  as  may  be  necessary  to  render 
function  clear,  but  we  think  these  few  generalizations  may  tend 
to  widen  the  student's  field  of  view,  and  at  the  same  time  lessen 
his  labor  and  render  it  more  effective. 

THE   MOVEMENTS    OF   THE   DIGESTIVE    ORGANS. 

As  with  other  parts  of  the  body,  so  in  the  alimentary  tract, 
the  slower  kind  of  movement  is  carried  out  by  plain  muscular 
fibers  ;  and  the  movements,  as  a  whole,  belong  to  the  class 
known  as  peristaltic  ;  in  fact,  it  is  only  at  the  beginning  of  the 
digestive  tract  that  voluntary  (striped)  muscle  is  to  be  found 
and  to  a  limited  extent  in  the  part  next  to  this— i.  e.,  in  the 
oesophagus. 

Teeth  in  the  highly  organized  mammal  are  remarkable  in 
being  to  the  least  degree  living  structures  of  any  in  the  entire 
animal,  thus  being  in  marked  contrast  to  other  organs.  The 
enamel  covering  their  exposed  surfaces  is  the  hardest  of  all  the 
tissues,  and  is  necessarily  of  low  vitality.  We  have  already 
alluded  to  the  difference  in  the  teeth  of  different  animals,  and 
their  relation  to  customary  food  and  digestive  functions.  In 
fact,  it  is  clear  that  the  teeth  and  all  the  parts  of  the  digestive 
system  are  correlated  to  one  another.  The  compound  stomach 
of  the  ruminants,  Avith  its  slow  digestion  of  a  bulky  mass  of 
food  which  must  be  softened  and  thoroughly  masticated  be- 
fore the' digestive  juices  can  attack  it  successfully,  harmonizes 
with  the  powerful  jaws,  strong  muscles  of  mastication,  and 
grinding  teeth  ;  and  all  these  in  marked  contrast  with  the  teeth 
of  a  carnivorous  animal  with  its  simple  but  highly  effective 
stomach.     Compare  figures  in  earlier  pages. 

Mastication  in  man  is  of  that  intermediate  character  befit- 
ting an  omnivorous  animal.  The  jaws  have  a  lateral  and  for- 
ward-and-backward  movement,  as  well  as  a  vertical  one,  though 
the  latter  is  predominant.  The  upper  jaw  is  like  a  fixed  mill- 
stone, against  which  the  lower  jaw  works  as  a  nether  millstone. 
The  elevation  of  the  jaw  is  effected  by  the  masseter,  temporal, 
and  internal  pterygoid  muscles  ;  depressed  by  the  mylohyoid 
and  geniohyoid,  though  principally  by  the  digastric.  The  jaw 
is  advanced  by  the  external  pterygoids;  unilateral  contraction 


DIGESTION   OF   FOOD.  333 

of  these  muscles  also  produces  lateral  movement  of  the  inferior 
maxilla,  which  is  retracted  by  the  more  horizontal  fibers  of  the 
temporal.  The  movements  of  mastication  are,  of  course,  very 
pronounced  in  ruminants. 

The  cheeks  and  tongue  likewise  take  part  in  preparing  the 
food  for  the  work  of  the  stomach,  nor  must  the  lips  be  over- 
looked even  in  man.  The  importance  of  these  parts  is  well 
illustrated,  by  the  imperfect  mastication,  etc.,  when  there  is 
paralysis  of  the  muscles  of  which  they  are  formed.  Even  when 
there  is  loss  of  sensation  only,  the  work  of  the  mouth  is  done 
in  a  clumsy  way,  showing  the  importance  of  common  sensation, 
as  well  as  the  muscular  sense. 

Nervous  Supply. — The  muscles  of  the  tongue  are  governed  by 
the  hypoglossal  nerve ;  the  other  muscles  of  mastication  chiefly 
by  the  fifth.  The  afferent  nerves  are  branches  of  the  fifth  and 
glossopharyngeal.  It  is,  of  course,  important  that  the  food 
should  be  rolled  about  and  thoroughly  mixed  with  saliva  (in- 
salivation). 

Deglutition. — The  transportation  of  the  food  from  the  mouth 
to  the  stomach  involves  a  series  of  co-ordinated  muscular  acts 
of  a  'complicated  character,  by  which  difficulties  are  overcome 
with  marvelous  success. 

It  will  be  remembered  that  the  respiratory  and  digestive 
tracts  are  both  developed  from  a  common  simple  tube — a  fact 
which  makes  the  close  anatomical  relation  between  these  two 
physiologically  distinct  systems  intelligible ;  but  it  also  involves 
difficulties  and  dangers.  It  is  well  known  that  a  small  quantity 
of  food  or  drink  entering  the  windpipe  produces  a  perfect 
storm  of  excitement  in  the  respiratory  system.  The  food,  there- 
fore, when  it  reaches  the  oesophagus,  must  be  kept,  on  the  one 
hand,  from  entering  the  nasal,  and  on  the  other,  the  laryngeal 
openings.  This  is  accomplished  as  follows:  When  the  food  has 
been  gathered  into  a  bolus  on  tbe  back  of  the  tongue,  the  tip  of 
this  organ  is  pressed  against  the  hard  palate,  by  which  the 
mass  is  prevented  from  passing  forward,  and,  at  the  same  time, 
forced  back  into  the  pharynx,  the  soft  palate  being  raised  and 
the  edges  of  the  pillars  of  the  fauces  made  to  approach  the 
uvula,  which  fills  up  the  gap  remaining,  so  that  the  posterior 
nares  are  closed  and  an  inclined  plane  provided,  over  which 
the  morsel  glides.  The  after-result  is  said  to  depend  on  the 
size  of  the  bolus.  When  considerable,  the  constrictors  of  the 
pharynx  seize  it  and  press  it  on  into  the  gullet ;  when  the  mor- 


334 


COMPARATIVE   PHYSIOLOGY. 


sel  is  small  or  liquid  is  swallowed,  it  is  rapidly  propelled  on- 
ward by  the  tongue,  the  oesophagus  and  pharynx  being  largely 
passive  at  the  time,  though  contracting  slowly  afterward;  at 


-■'■•'/         ■,■  ■'■■:   -    ■*/;*       ;A*      -a 


\\     '7£&:     .     /O 


Fig.  274.— Cavities  of  mouth  and  pharynx,  etc..  in  man  (after  Sappey).  Section,  in 
median  line,  of  face  and  superior  portion  of  neck,  designed  to  show  the  mouth  in 
its  relations  to  the  nasal  fossae,  pharynx,  and  larynx:  1,  sphenoidal  sinuses;  2,  in- 
ternal orifice  of  Eustachian  tube;  3,  palatine  arch;  4,  velum  pendulum  palati;  5. 
anterior  pillar  of  soft  palate;  6,  posterior  pillar  of  soft  palate;  7,  tonsil;  8,  lingual 
portion  of  cavity  of  pharynx;  9,  epiglottis;  10,  section  of  hyoid  bone;  11,  laryn- 
geal  portion  of  cavity  of  pharynx;   12,  cavity  of  larynx. 

the  same  time  the  larynx  as  a  whole  is  raised,  the  epiglottis 
pressed  down,  chiefly  by  the  meeting  of  the  tongue  and  itself, 
while  its  cushion  lies  over  the  rima  glottidis,  which  is  closed 
or  all  but  closed  by  the  action  of  the  sphincter  muscles  of  the 
larynx,  so  that  the  food  passes  over  and  by  this  avenue  of  life, 
not  only  closed  but  covered  by  the  glottic  lid.  The  latter  is 
not   so  essential  as  might  be  supposed,  for  persons  in  whom  it 


DIGESTION   OF   FOOD.  335 

was  absent  have  been  known  to  swallow  fairly  well.  The 
ascent  of  the  larynx  any  one  may  feel  for  himself  ;  and  the  be- 
havior of  the  pharynx  and  larynx,  especially  the  latter,  may 
be  viewed  by  the  laryngoscope.  The  grip  of  the  pharyngeal 
muscles  and  the  oesophagus  may  be  made  clear  by  attaching  a 
piece  of  food  (meat)  to  a  string  and  allowing  it  to  be  partially 
swallowed. 

The  upward  movement  of  food  under  the  action  of  the 
constrictors  of  the  pharynx  is  anticipated  by  the  closure  of 
the  passage  by  the  palato-glossi  of  the  anterior  pillors  of  the 
fauces. 

The  circular  muscular  fibers  of  the  gullet  are  probably  the 
most  important  in  squeezing  on  the  food  by  a  peristaltic  move- 
ment, passing  progressively  over  the  whole  tube,  though  the 
longitudinal  also  take  part  in  swallowing,  perhaps,  by  steady- 
ing the  organ. 

Deglutition  can  take  place  in  an  animal  so  long  as  the 
medulla  oblongata  remains  intact ;  and  the  center  seems  to  lie 
higher  than  that  for  respiration,  as  the  latter  act  is  possible 
when,  from  slicing  away  the  medulla,  the  former  is  not.  An- 
encephalous  monsters  lacking  the  cerebrum  can  swallow,  suck, 
and  breathe. 

Food  placed  in  the  pharynx  of  animals  when  unconscious 
is  swallowed,  proving  that  volition  is  not  essential  to  the  act ; 
but  our  own  consciousness  declares  that  the  first  stage,  or  the 
removal  of  the  food  from  the  mouth  to  the  pharynx,  is  volun- 
tary. 

When  we  seem  to  swallow  voluntarily  there  is  in  reality  a 
stimulus  applied  to  the  fauces,  in  the  absence  of  food  and  drink, 
either  by  the  back  of  the  tongue  or  by  a  little  saliva. 

It  thus  appears  that  deglutition  is  an  act  in  the  main  reflex, 
though  initiated  by  volition.  The  afferent  nerves  concerned 
are  usually  the  glossopharyngeal,  some  branches  of  the  fifth, 
and  of  the  vagus.  The  efferent  nerves  are  those  of  the  numer- 
ous muscles  concerned. 

When  food  has  reached  the  gullet  it  is,  of  course,  no  longer 
under  the  control  of  the  will. 

Section  of  the  vagus  or  stimulation  of  this  nerve  modifies 
the  action  of  the  oesophagus,  though  it  is  known  that  contrac- 
tions may  be  excited  in  the  excised  organ  ;  but  no  doubt  nor- 
mally the  movements  of  the  gullet  arise  in  response  to  natural 
nerve  stimulation. 


336 


comparative;  physiology. 


Comparative. — That  swallowing  is  independent  of  gravity  is 
evident  from  the  fact  that  long-necked  animals  (horse,  giraffe) 
can  and  do  usually  swallow  with  the  head  and  neck  down,  so 
that  the  fluid  is  rolled  up  an  inclined  plane.  The  peristaltic 
nature  of  the  contractions  of  the  gullet  can  also  be  well  seen  in 
such  animals.  In  the  frog  the  gullet,  as  well  as  the  mouth,  is 
lined  with  ciliated  epithelium,  so  that  in  a  recently  killed  ani- 
mal one  may  watch  a  slice  of  moistened  cork  disappear  from  the 
mouth,  to  be  found  shortly  afterward  in  the  stomach.  The  rate 
of  the  descent  is  surprising — in  fact,  the  movement  is  plainly 
visible  to  the  unaided  eye. 

The  Movements  of  the  Stomach. — The  stomach  of  mammals, 
including  man,  is  provided  with  three  layers  of  muscular  fibers ; 
1.  External  longitudinal,  a  continuation  of  those  of  the  oesopha- 
gus. 2.  Middle  circular.  3.  Internal  oblique.  The  latter  are 
the  least  perfect,  viewed  as  an  investing  coat.  The  pyloric  end 
of  the  stomach  is  best  supplied  with  muscles ;  where  also  there 
is  a  thick  muscular  ring  or  sphincter,  as  compared  with  which 
the  cardiac  sphincter  is  weak  and  ill-developed. 


Fig.  275. 


Pig.  276. 


Pig  275  —Muscular  fibers  of  the  stomach  of  horse;  external  and  middle  layers  (Chau- 
veau).  A,  fillers  of  external  layer  enveloping  left  sac;  B,  fibers  of  middle  plane 
in  right  sac:  C,  fibers  of  oesophagus. 

F„;  270  —Deep  and  muscular  layers  exposed  by  removing  mucous  membrane  from  an 
everted  stomach  (Chau'veau).  A,  deep  layer  of  libers  enveloping  left  sac;  B,  fibers 
of  middle  plane  which  alone  form  the  muscular  layer  of  right  sac;  C,  fibers  of 
oesophagus. 


The  movements  of  the  stomach  begin  shortly  after  a  meal 
has  been  taken,  and,  as  shown  by  observations  on  St.  Martin, 
continue  for  hours,  not  constantly,  but  periodically.    The  effect 


DIGESTION   OF   FOOD.  337 

of  the  conjoint  action  of  the  different  sets  of  muscular  fibers  is 
to  move  the  food  from  the  cardiac  toward  the  pyloric  end  of 
the  stomach,  along1  the  greater  curvature  and  back  by  the  lesser 
curvature,  while  there  is  also,  probably,  a  series  of  in-and-out 
currents  to  and  from  the  center  of  the  food-mass.  The  quantity 
of  food  is  constantly  being  lessened  by  the  removal  of  digested 
portions,  either  by  the  blood-vessels  of  the  organ  or  by  its 
passing  through  the  pyloric  sphincter.  The  empty  stomach  is 
quiescent  and  contracted,  its  mucous  membrane  being  thrown 
into  folds. 

The  movements  of  the  stomach  may  be  regarded  as  reflex, 
the  presence  of  food  being  an  exciting  cause,  though  probably 
not  the  only  one  ;  and  so  largely  automatic  is  the  central  mech- 
anisms concerned  that  but  a  feeble  stimulus  suffices  to  arouse 
them,  especially  at  the  accustomed  time. 

Of  the  paths  of  the  impulses,  either  afferent  or  efferent, 
little  is  known.  Certain  effects  follow  section  or  stimulation  of 
the  vagi  or  splanchnics,  but  these  can  not  be  predicted  with 
certainty,  or  the  exact  relation  of  events  indicated. 

It  is  said  that  the  movements  of  the  stomach  cease,  even 
when  it  is  full,  during  sleep,  from  which  it  is  argued  that  gas- 
tric movements  do  normally  depend  on  the  influence  of  the 
nervous  system.  However,  the  subject  is  too  obscure  at  present 
for  further  discussion. 

Comparative. — Recent  investigations  on  the  stomach  of  the 
pig  indicate  that  in  this  animal  the  contents  of  the  two  ends  of 
the  stomach  may  long  remain  but  little  mingled  ;  and  such  is 
certainly  the  case  in  this  organ  among  ruminants. 

Pathological.— Distention  of  the  stomach,  either  from  excess 
of  food  or  gas  arising  from  fermentative  changes,  or  by  secre- 
tion from  the  blood,  may  cause,  by  upward  pressure  on  the 
diaphragm,  etc.,  uneasiness  from  hampered  respiration  and 
irregularity  of  the  heart,  possibly,  also,  in  part  traceable  to  the 
physical  interference  with  its  movements.  After  gx*eat  and 
prolonged  distention  there  may  be  weakened  digestion  for  a 
considerable  interval.  It  seems  not  improbable  that  this  is  to 
be  explained,  not  alone  by  the  impaired  elasticity  (vitality)  of 
the  muscular  tissue,  but  also  by  defective  secreting  power.  It 
is  not  necessary  to  impress  the  lesson  such  facts  convey. 

The  Intestinal  Movements.— The  circular  fibers  play  a  much 
more  important  part  than  the  longitudinal,  being,  in  fact,  much 
more  developed.     It  is  also  to  be  remembered  that  nerves  in 


338  COMPARATIVE  PHYSIOLOGY. 

the  form  of  plexuses  (of  Auerbach  and  Meissner)  abound  in  its 
walls. 

Normally  the  movement,  slowly  progressive,  with  occasional 
haltings,  is  from  above  downward,  stopping  at  the  ileo-caecal 
valve ;  the  movements  of  the  large  gut  being  apparently  mostly 
independent. 

Movements  may  be  excited  by  external  or  internal  stimula- 
tion, and  may  be  regarded  as  reflex;  in  which,  however,  the 
tendency  for  the  central  cells  to  discharge  themselves  is  so  great 
(automatic)  that  only  a  feeble  stimulus  is  required,  the  normal 
one  being  the  presence  of  food. 

It  is  noticeable  in  a  recently  killed  animal,  or  in  one  in  the 
last  stages  of  asphyxia,  that  the  intestines  contract  vigorously. 
Whether  this  is  due  to  the  action  of  blood  overcharged  with 
carbonic  anhydride  and  deficient  in  oxygen  on  the  centers  pre- 
siding over  the  movements,  on  the  nerves  in  the  intestinal 
walls,  or  on  the  muscle-cells  directly,  is  not  wholly  clear,  but  it 
is  probable  that  all  of  these  may  enter  into  the  result.  The 
vagus  nerve,  when  stimulated,  gives  rise  to  movements  of  the 
intestines,  while  the  splanchnic  seems  to  have  the  reverse  effect  ; 
but  the  cerebrum  itself  has  an  influence  over  the  movements  of 
the  gut,  as  is  plain  from  the  diarrhoea  traceable  to  unusual  fear 
or  anxiety.  There  is  little  to  add  in  regard  to  the  movements 
of  the  large  intestine.  They  are,  no  doubt,  of  considerable  im- 
portance in  animals  in  which  it  is  extensive.  Normally  they 
begin  at  the  ileo-caecal  valve. 

Defecation. — The  removal  of  the  waste  matter  from  the  ali- 
mentary tract  is  a  complicated  process,  in  which  both  smooth 
and  striped  muscle,  the  spinal  cord,  and  the  brain  take  part. 

Defecation  may  take  place  during  the  unconsciousness  of 
sleep  or  of  disease,  and  so  be  wholly  independent  of  the  will  ; 
but,  as  we  all  know,  this  is  not  usually  the  case.  Against  ac- 
cidental discharge  of  faeces  there  is  a  provision  in  the  sphinc- 
ter ani,  the  tone  of  which  is  lost  when  the  lower  part  of  the 
spinal  cord  is  destroyed.  We  are  conscious  of  being  able,  by  an 
effort  of  will,  to  prevent  tbe  relaxation  of  the  sphincter  or  to 
increase  its  holding  power,  though  the  latter  is  probably  almost 
wholly  due  to  the  action  of  extrinsic  muscles  ;  at  all  events  any 
one  may  convince  himself  that  the  latter  may  be  made  to  take 
a  great  part  in  preventing  faecal  discharge,  though  whether  the 
tone  of  the  sphincter  can  be  increased  or  not  by  volition  it  is 
difficult  to  say. 


DIGESTION   OF  FOOD.  339 

What  happens  during  an  ordinary  act  of  defecation  is  about 
as  follows  :  After  a  long  inspiration  the  glottis  is  closed  ;  the 
diaphragm,  which  has  descended,  remains  low,  affording,  with 
the  obstructed  laryngeal  outlet,  a  firm  basis  of  support  for  the 
action  of  the  abdominal  muscles,  which,  bearing  on  the  intes- 
tine, forces  on  their  contents,  which,  before  the  act  has  been 
called  for,  have  been  lodged  mostly  in  the  large  intestine  ;  at 
the  same  time  the  sphincter  ani  is  relaxed  and  peristaltic  move- 
ments accompany  and  in  some  instances  precede  the  action  of 
the  abdominal  muscles.  The  latter  may  contract  vigorously  on 
a  full  gut  without  success  in  the  absence  of  the  intestinal  peri- 
stalsis, as  too  many  cases  of  obstinate  constipation  bear  witness. 

Like  deglutition,  and  unlike  vomiting,  there  is  usually  both 
a  voluntary  and  involuntary  part  to  the  act. 

Though  the  will,  through  the  cerebrum,  can  inhibit  defeca- 
tion, it  is  likely  that  it  does  so  through  the  influence  of  the 
cerebrum  on  some  center  in  the  cord  ;  for  in  a  dog,  the  lumbar 
cord  of  which  has  been  divided  from  the  dorsal,  the  act  is,  like 
micturition,  erection  of  the  penis,  and  others  which  are  under 
the  control  of  the  will,  still  possible,  though,  of  course,  per- 
formed entirely  unconsciously. 

Vomiting. — If  we  consult  our  own  consciousness  and  observe 
to  the  best  of  our  ability,  supplementing  information  thus 
gained  by  observations  on  others  and  on  the  lower  animals,  it 
will  become  apparent  that  vomiting  implies  a  series  of  co-ordi- 
nated movements  into  which  volition  does  not  enter  either 
necessarily  or  habitually.  There  is  usually  a  preceding  nausea, 
with  a  temporary  flow  of  saliva  to  excess.  The  act  is  initiated 
by  a  deep  inspiration,  followed  by  closure  of  the  glottis. 
Whether  the  glottis  is  closed  during  or  prior  to  the  entrance 
of  air  is  a  matter  of  disagreement.  At  all  events,  the  dia- 
phragm descends  and  remains  fixed,  the  lower  ribs  being  re- 
tracted. The  abdominal  muscles  then  acting  against  this  sup- 
port, force  out  the  contents  of  the  stomach,  in  which  they  are 
assisted  by  the  essential  relaxation  of  the  cardiac  sphincter,  the 
shortening  of  the  oesophagus  by  its  longitudinal  fibers,  and  the 
extension  and  straightening  of  the  neck,  together  with  the  open- 
ing of  the  mouth. 

As  the  expulsive  effort  takes  place,  it  is  accompanied  by  an 
expiratory  act  which  tends  to  keep  the  egesta  out  of  the  larynx 
and  carry  them  onward,  though  it  may  also  contribute  to  over- 
come the  resistance  of  the  elevated  soft  palate,  which  serves  to 


340  COMPARATIVE  PHYSIOLOGY. 

protect  the  nasal  passages.  The  stomach  and  oesophagus  are 
not  wholly  passive,  though  the  part  they  take  actively  in  vom- 
iting is  variable  in  different  animals. 

Retching  may  be  very  violent  and  yet  ineffectual  when  the 
cardiac  sphincter  is  not  fully  relaxed.  The  pyloric  outlet  is 
usually  closed,  though  in  severe  and  long-continued  vomiting 
bile  is  often  ejected,  which  must  have  reached  the  stomach 
through  the  pylorus. 

Comparative. — The  ease  with  which  some  animals  vomit  in 
comparison  with  others  is  extraordinary,  as  in  carnivora  like 
our  dogs  and  cats  ;  a  matter  of  importance  to  an  animal  accus- 
tomed in  the  wild  state  to  eat  entire  carcasses  of  animals — hair, 
bones,  etc.,  included. 

The  readiness  with  which  an  animal  vomits  depends  in  great 
part  on  the  conformation  and  relations  of  the  parts  of  its  digest- 
ive tract. 

The  stomach  of  the  human  being  during  infantile  life  is  less 
pouched  than  in  the  adult,  which  in  part  explains  the  ease  with 
which  very  young  children  vomit. 

It  is  well  known  that  the  horse  vomits  rarely  and  with  great 
difficulty.  This  has  been  attributed  by  different  writers  to  va- 
rious conditions  of  a  structural  kind,  such  as  the  length  of  the 
gullet  ;  the  manner  in  which  it  enters  the  stomach  (centrally)  ; 
the  pressure  of  a  tightly  closing  sphincter  at  this  point  ;  the 
valve-like  foldings  of  the  mucous  membrane  at  the  cardiac 
opening  ;  the  small  size  of  the  stomach  and  its  sheltered  posi- 
tion, so  that  the  abdominal  muscles  can  not  readily  act  on  it  ; 
the  existence  of  a  considerable  length  of  the  oesophagus  be- 
tween the  stomach  and  diaphragm  which  is  against  dilatation 
of  the  orifice  by  the  longitudinal  fibers  of  the  gullet  ;  the  open 
pylorus,  permitting  of  the  gastric  contents  being  driven  into  the 
intestines  rather  than  upward. 

But  in  the  ox  these  peculiarities  do  not  exist;  in  fact,  from  a 
mechanical  point  of  view,  the  stracture  and  illation  of  parts  is 
favorable,  yet  this  animal  seldom  vomits,  and  never  with  ease. 
Why  does  the  horse  vomit  after  rupture  of  the  stomach  when 
conditions  are  less  favorable  from  a  mechanical  point  of  view  ? 
There  is  the  greatest  difference  as  to  the  readiness  with  which 
different  human  beings  vomit  ;  moreover,  persons  that  vomit 
usually  with  difficulty  may  do  so  very  perfectly  when  suffi- 
ciently prepared,  as  by  sea-sickness. 

These  and  many  other  considerations  have  led  us  to  conclude 


DIGESTION   OF   FOOD.  341 

that,  while  there  is  a  certain  amount  of  force  in  the  various 
views  stated  briefly  above,  they  do  not  go  to  the  root  of  the 
matter. 

Vomiting  is  a  very  complex  act,  implying  numerous  muscu- 
lar and  nervous  co-oi'dinations.  In  the  natural  wild  state  the 
horse  can  have  but  rare  necessity  to  vomit  (unlike  the  carnivora), 
hence  these  co-ordinations  have  not  been  organized  by  habit 
and  use  ;  they  are  foreign  to  the  nature  of  the  animal.  After 
rupture  of  the  stomach  in  the  horse,  and  in  sea-sickness  in  man, 
the  nervous  system  is  profoundly  affected  and  the  unusual  hap- 
pens ;  in  other  words,  the  necessary  muscular  and  nervous 
co-ordinations  take  place.  At  all  events,  we  are  satisfied  that  it 
lies  with  the  nervous  system  chiefly. 

Similarly,  the  habit  of  regurgitating  the  food  is  intelligible 
in  the  light  of  evolution.  The  fact  that  mammals  are  descended 
from  lower  forms  in  which  unstriped  muscle-cells  go  to  form 
organs  that  have  a  rhythmically  contractile  function,  renders 
it  clear  why  this  function  may  become,  as  in  ruminants,  spe- 
cialized in  certain  parts  of  the  digestive  tract ;  why  carnivora 
should  vomit  readily,  and  why  human  subjects  should  learn  to 
regurgitate  food.  There  is,  so  to  speak,  a  latent  inherited  ca- 
pacity which  may  be  developed  into  actual  function.  Apart 
from  this  it  is  difficult  to  understand  such  cases  at  all. 

The  vomiting  center  is  usually  located  in  the  medulla,  and 
is  represented  as  working  in  concert  with  the  respiratory  center. 
But  when  we  consider  that  there  is  usually  an  increased  flow 
of  saliva  and  other  phenomena  involving  additional  central 
nervous  influence,  we  see  reason  to  believe  in  co-ordinated 
action  implying  the  use  of  parts  of  the  central  nervous  system 
not  so  closely  connected  anatomically  as  the  respiratory  and 
vomiting  centers  are  assumed  to  be. 


THE   REMOVAL   OF   DIGESTED  PRODUCTS  FROM  THE 
ALIMENTARY   CANAL. 

The  glands  of  the  stomach  are  simply  seci-etive,  and  all  ab- 
sorption from  this  organ  is  either  by  blood-vessels  directly  or 
by  lymphatics ;  at  least,  such  is  the  ordinary  view  of  the  subject 
— whether  it  is  not  too  narrow  a  one  remains  to  be  seen. 

It  is  important  to  remember  that  the  intestinal  mucous 
membrane  is  supplied  not  only  with  secreting  glands  but  lym- 
phatic tissue,  m  the  form  of  the  solitary  and  agmiiiated  glands 


3±2 


COMPARATIVE   PHYSIOLOGY. 


(Peyer's  patches)  and  thickly  studded  with  villi,  giving  the 
small  gut  that  velvety  appearance  appreciable  even  by  the 
naked  eye. 

It  will  not  be  forgotten  that  the  capillaries  of  the  digestive 
organs  terminate  in  the  veins  of  the  portal  system,  and  that  the 
blood  from  these  parts  is  conducted  through  the  liver  before  it 
reaches  the  general  circulation. 


Main  venous  trunk 


Right  auricle 


Vena  cava 


Hepatic  vein 


Lymph,  gland 


Portal  system 


W  @0    Blood  vessel,  tissue  cells. 


ilimentary  tract 


Fig.  277.— Diagram  intended  to  illustrate  the  general  relations  of  blood  and  lymph  to 
metabolism  (nutrition),  and  the  method  by  which  the  portal,  lymphatic,  and  gen- 
eral venous  systems  are  related  to  the  alimentary  tract. 

The  lymphatics  of  these  organs  form  a  part  of  the  general 
lymphatic  system  of  the  body ;  but  the  peculiar  way  in  which 
absorption  is  effected  by  villi,  and  the  fact  that  the  lymphatics 
of  the  intestine,  etc.,  at  one  time  (fasting)  contain  ordinary 
lymph  and  at  another  (after  meals)  the  products  of  digestion, 
imparts  to  them  a  physiological  character  of  their  own. 

Absorption  will  be  the  better  understood  if  we  treat  now  of 
lymph  and  chyle  and  the  lymph  vascular  system,  which  were 
purposely  postponed  till  the  present;  though  its  connection 
with  the  vascular  system  is  as  close  and  important  as  with  the 
digestive  organs. 

The  lymphatic  system,  as  a  whole,  more  closely  resembles 
the  venous  than  the  arterial  vessels.  We  may  speak  of  lym- 
phatic capillaries,  which  are,  in  essential  points  of  structure, 
like  the  arterial  capillaries;  while  the  larger  vessels  may  be 
compared  to  veins,  though  thinner,  being  provided  with  valves 
and   having  very   numerous  anastomoses.      These    lymphatic 


DIGESTION  OF  FOOD, 


343 


capillaries  begin  in  spaces  between  tbe  tissue- 
cells,  from  wbicb  they  take  up  the  effete 
lymph.  It  is  interesting  to  note  that  there 
are  also  perivascular  lymphatics,  the  exist- 
ence of  which  again  shows  how  close  is  the 
relation  between  the  blood  vascular  and  lym- 
phatic systems,  and  as  we  would  suppose,  and 
as  is  actually  found  to  be  the  case,  between 
the  contents  of  each. 

Lymph  and  Chyle.— If  one  compares  the 
mesentery  in  a  kitten  when  fasting  with  the 
same  part  in  an  animal  that  was  killed  some 
hours  after  a  full  meal  of  milk,  it  may  be  seen 
that  the  formerly  clear  lines  indicating  the 
course  of  the  lymphatics  and  ending  in  glands 
have  in  the  latter  case  become  whitish  (hence 
their  name,  lacteals),  owing  to  the  absorp- 
tion of  the  emulsified  fat  of  the  milk. 

Microscopic  examination  shows  the  chyle 
to  contain   (when  coagulated)    fibrin,  many 


Fig.  278.— Valves 
of  lymphatics 
(Sappey). 


Fig.  279.— Origin  of  lymphatics  (after  Landois).  I.  From  central  tendon  of  diaphragm 
of  rabbit  (semi-diagrammatic) ;  6'.  lymph-canals  communicating  by  X  with  lym- 
phatic vessel  L\  A,  origin  of  lymphatic  by  union  of  lymph-canals;  E,  E.  endothe- 
lium.   II.  Perivascular  canal. 


344 


COMPARATIVE   PHYSIOLOGY. 


leucocytes,  a  few  developing  red  corpuscles,  an  abundance  of 
fat  in  the  form  both  of  very  minute  oil-globules  and  particles 
smaller  still. 


Fig.  280. — Epithelium  from  duodenum  of 
rabbit,  two  hours  after  having  been  fed 
with  melted  butter  (Funke). 


Fig.  281.— Villi  filled  with  fat,  from 
small  intestine  of  an  executed  crim 
inal,  one  hour  after  death  (Funke). 


There  are  also  present  fatty  acids,  soaps  small  in  quantity 
as  compared  with  the  neutral  fats,  also  a  little  cholesterin  and 
lecithin.  But  chyle  varies  very  widely  even  in  the  same  animal 
at  different  times.  To  the  above  must  be  added  proteids  (fibrin, 
serum-albumin,  and  globulin) ;  extractives  (sugar,  urea,  leucin) ; 

and  salts  in  which  sodium 
chloride  is  abundant. 

The  composition  of 
lymph  is  so  similar  to  that 
of  chyle,  and  both  to  blood, 
that  lymph  might,  'though 
only  roughly,  be  regarded 
as  blood  without  its  red  cor- 
puscles, and  chyle  as  lymph 
with  much  neutral  fat  in  a 
very  fine  state  of  division. 

The  Movements  of  the 
Lymph  —  comparative.  —  In 
some  fishes,  some  birds,  and 

:.  882.— Chyle  taken  from  the  lacteals  and  amphibians,  there  are  lymph 
thoracic  duct,  of  a  criminal  executed  dur- 

ing  digestion  (Funke).    shown  leucocytes  hearts, 
and  excessively  fine   granules    of   fatty  In  the  frog  there  are  two 


Fir 


DIGESTION   OP   FOOD.  345 

axillary  and  two  sacral  lymph  hearts.  The  latter  are,  espe- 
cially, easily  seen,  and.  there  is  no  doubt  that  they  are  under 
the  control  of  the  nervous  system. 

In  the  mammals  no  such  special  helps  for  the  propulsion  of 
lymph  exist. 

There  is  little  doubt  that  the  blood-pressure  is  always  higher 
than  the  lymph-pressure,  and  when  the  blood-vessels  are  dilated 
the  fluid  within  the  perivascular  lymph-channels  is  likely  com- 
pressed; muscular  exercise  must  act  on  the  lymph-channels  as 
on  veins,  both  being  provided  with  valves,  though  themselves 
readily  compressible;  the  inspiratory  efforts,  especially  when 
forcible,  assist  in  two  ways:  by  the  compressing  effect  of  the 
respiratory  muscles,  and  by  the  aspirating  effect  of  the  negative 
pressure  within  the  thorax,  producing  a  similar  aspirating 
effect  within  the  great  veins,  into  which  the  large  lymphatic 
trunks  empty.  The  latter  are  provided  at  this  point  with 
valves,  so  that  there  is  no  back-flow;  and,  with  the  positive 
pressure  within  the  large  lymphatic  trunks  (thoracic  duct,  etc.), 
the  physical  conditions  are  favorable  to  the  outflow  of  lymph 
or  chyle. 

Our  knowledge  of  the  nature  of  the  passage  of  the  chyle 
from  the  intestines  into  the  blood  is  now  clearer  than  it  was  till 
recently,  though  still  incomplete. 

The  exact  structure  of  a  villus  is  to  be  carefully  considered. 
If  we  assume  that  the  muscular  cells  in  its  structure  have  a 
rhythmically  contractile  function,  the  blind  terminal  portion 
of  the  lacteal  inclosed  within  the  villus  must,  after  being 
emptied,  act  as  a  suction -pump  to  some  extent;  at  all  events, 
the  conditions  as  to  pressure  would  be  favorable  to  inflow  of 
any  material,  especially  fluid  without  the  lacteal.  The  great 
difficulty  hitherto  was  to  understand  how  the  fat  found  its  way 
through  the  villus  into  the  blood,  for,  that  most  of  it  passes  in 
this  direction  there  is  little  doubt. 

It  is  now  known  that  leucocytes  (amceboids,  phagocytes) 
migrate  from  within  the  villus  outward,  and  may  even  reach 
its  surface,  that  they  take  up  (eat)  fat-particles  from  the  epi- 
thelium of  the  villus,  and,  independently  themselves,  carry 
them  inward,  reach  the  central  lacteal  and  break  up,  thus  re- 
leasing the  fat.  How  the  fat  gets  into  the  covering  epithelium 
is  not  yet  so  fully  known — possibly  by  a  similar  inceptive  pro- 
cess; nor  is  it  ascertained  what  constructive  or  other  chemical 
processes  it  may  perform;  though  it  is  not  at  all  likely  that 


Pig.  288 


DIGESTION   OF   FOOD. 


347 


Fig.  283.— Lymphatic  system  of  horse  (Chauveau).  A,  facial  and  nasal  plexus  whose 
branches  pass  to  subglossal  glands;  B,  C,  parotid  lymphatic  gland,  sending  ves- 
sels to  pharyngeal  gland;  D.  E,  large  trunks  passing  toward  thorax;  P.  G,  H, 
glands  receiving  superficial  lymphatics  of  neck,  a  portion  of  those  of  limbs,  and 
those  of  pectoral  parietes;  I,  junction  of  jugulars;  J,  axillary  veins;  K,  summit 
of  anterior  vena  cava;  L,  thoracic  duct;  M,  lymphatics  of  spleen;  in',  of  stomach; 
O,  of  large  colon;  S,  of  small  colon;  R,  lacteals  of  small  intestine,  all  going  to 
form  two  trunks,  P.  Q,  which  open  directly  into  receptaculum  chyli;  T,  trunk 
which  receives  branches  of  sub  lumbar  glands.  U,  to  which  vessels  of  internal  iliac 
glands,  V,  the  receptacles  of  lymphatics  of  abdominal  parietes,  pass;  W,  precrural 
glands  receiving  lymphatics  of  posterior  limb,  and  which  arrive  independently  in 
the  abdomen;  S,  superficial  inguinal  glands  into  which  lymphatics  of  the  mam- 
mce,  external  generative  organs,  some  superficial  trunks  of  posterior  limb,  etc., 
pass;  Z,  deep  inguinal  glands  receiving  the  superficial  lymphatics,  Z,  of  posterior 
limbs. 

the  work  of  the  amoeboid  cells  is  confined  to  the  transport  of  fat 
alone,  but  that  other  matters  are  also  thus  removed  inward  to 
the  lacteal. 

When  a  multitude  of  facts  are  taken  into  account,  thei^e 


Fig.  284.— Perpendicular  section  through  one  of  Peyer's  patches  in  the  lower  part  of 
the  ileum  of  the  sheep  (Chauveau).  a,  a,  lacteal  vessels  in  villi;  b,  b,  superficial 
layer  of  lacteal  vessels;  c,  c,  deep  layer  of  lacteals;  d.  cl,  efferent  vessels  provided 
with  valves;  /,  Peyer's  glands;  g,  circular  muscular  layer  of  wall  of  intestine;  /;, 
longitudinal  layer. 


seems  little  reason  to  doubt  that  so  important  a  process  as  ab- 
sorption can  not  fail  to  be  regulated  by  the  nervous  centers. 


348 


COMPARATIVE  PHYSIOLOGY. 


There  are  two  points  that  are  very  far  from  being  deter- 
mined :  the  one  the  fate  of  the  products  of  digestion ;  the  other 
the  exact  limit  to  which  digestion  is  carried.  How  much — e.  g., 
of  proteid  matter — does  actually  undergo 
conversion  into  peptone;  how  much  is 
converted  into  leucin  and  tyrosin;  or, 
again,  what  proportion  of  the  albuminous 
matters  are  dealt  with  as  such  by  the  in- 
testine without  conversion  into  peptone 
at  all,  either  as  soluble  proteid  or  in  the 
form  of  solid  particles  ? 

1.  It  is  generally  believed  that  solu- 
ble sugars  are  absorbed,  usually  after 
conversion  into  maltose  or  glucose,  by 
the  capillaries  of  the  stomach  and  intes- 
tine. 

2.  There  is  some  positive  evidence  of 
the  presence  of  fats,  soaps,  and  sugars  in 
unusual  amount  after  a  meal  in  the  por- 
tal vein,  which  implies  removal  from  the 
intestinal  contents  by  the  capillaries, 
though,  so  far  as  experiment  goes,  the 
fat  is  chiefly  in  the  form  of  soaps, 

Certain  experiments  have  been  made 
by  ligating  the  pyloric  end  of  the  stom- 
ach, by  introducing  a  cannula  into  the 
thoracic  duct,  so  as  to  continually  remove  its  contents,  etc. 
But  we  are  surprised  that  serious  conclusions  should  have  been 
drawn  under  such  circumstances,  seeing  that  the  natural  condi- 
tions are  so  altered.  What  we  wish  to  get  at  in  physiology  is 
the  normal  function  of  parts,  and  not  the  possible  results  after 
our  interference.  Under  such  circumstances  the  phenomena 
may  have  a  suggestive  but  certainly  can  not  have  a  conclusive 
value. 

It  is  a  very  striking  fact  that  little  peptone  (none,  according 
to  some  observers)  can  be  detected  even  in  the  portal  blood. 
True  it  is,  the  circulation  is  rapid  and  constant,  and  a  small 
quantity  might  escape  detection,  yet  a  considerable  amount  be 
removed  from  the  intestine  in  the  space  of  a  few  hours  by  the 
capillaries  alone.  Peptone  is  not  found  in  the  contents  of  the 
thoracic  duct. 

For  a  considerable  period  it  has  been  customary  to  use  the 


Fig 


.  285.  —  Intestinal  villus 
(after  Leydig).  a,  a,  a, 
epithelial  covering;  b,b, 
capillary  network;  c,  c, 
longitudinal  muscular 
fibers;  d,  lacteal. 


DIGESTION   OF   FOOD. 


349 


terms  osmosis  and  diffusion  in  connection  with  the  functions 
of  the  alimentary  canal,  and  especially  the  intestinal  tract, 
as  if  this  thin-walled  but  complicated  organ,  or  rather  collec- 


Fig.  286. — A.  Villi  of  man.  showing  blood-vessels  and  lacteals;  B.  Villus  of  sheep 
(after  Chauveau). 

tion  of  organs,  were  little  more,  so  far  as  absorption  is  con- 
cerned, than  a  moist  membrane,  leaving  the  process  of  the  re- 
moval of  digested  food  products  to  be  explained  almost  wholly 
on  physical  principles. 

From  such  views  we  dissent.  We  believe  they  are  opposed 
to  what  we  know  of  living  tissue  everywhere,  and  are  not  sup- 
ported by  the  special  facts  of  digestion.  When  certain  foreign 
bodies  (as  purgatives)  are  introduced  into  the  blood  or  the  ali- 
mentaiy  canal,  that  diffusion  takes  place,  according  to  physical 
laws,  may  indicate  the  manner  in  which  the  intestine  can  act; 
but  even  admitting  that  under  such  circumstances  physical 
principles  actually  do  explain  the  whole,  which  we  do  not  grant, 
it  would  by  no  means  follow  that  such  was  the  natural  behav- 
ior of  this  organ  in  the  discharge  of  its  ordinary  functions. 


350 


COMPARATIVE   PHYSIOLOGY. 


When  we  consider  that  the  blood  tends  to  maintain  an  equi- 
librium, it  must  be  evident  that  the  removal  of  substances  from 
the  alimentary  canal,  unless  there  is  to  be  excessive  activity  of 


str 


Fig.  287.— A.  Section  of  villus  of  rat  killed  during  fat  absorption  (Schafer).  ep,  epi- 
thelium; str,  striated  border;  c,  lymph-cells;  c',  lymph-cells  in  epithelium;  /,  cen- 
tral lacteal  containing  disintegrating  corpuscles.  B.  Mucous  membrane  of  frog's 
intestine  during  fat  absorption  (Schafer).  ep,  epithelium;  str,  striated  border;  C, 
lymph-corpuscles;  I,  lacteal. 

the  excretory  organs  and  waste  of  energy  both  by  them  and 
the  digestive  tract,  must  in  some  degree  depend  on  the  demand 
for  the  products  of  digestion  by  the  tissues.  That  there  is  to 
some  extent  a  corrective  action  of  the  excretory  organs  always 
going  on  is  no  doubt  true,  and  that  it  may  in  cases  of  emergency 
be  great  is  also  true  ;  but  that  this  is  minimized  in  ways  too 
complex  for  us  to  follow  in  every  detail  is  equally  true.  Diges- 
tion waits  on  appetite,  and  the  latter  is  an  expression  of  the 
needs  of  the  tissues.  We  believe  it  is  literally  true  that  in  a 
healthy  organism  the  rate  and  character  of  digestion  and  of 
the  removal  of  prepared  products  are  largely  dependent  on  the 
condition  of  the  tissues  of  the  body. 

Why  is  digestion  more  perfect  in   overfed   animals  after 
a  short  fast  ?    The  whole  matter  is  very  complex,  but  we  think 


DIGESTION   OF   FOOD.  351 

it  is  infinitely  better  to  admit  ignorance  than  attempt  to  ex- 
plain by  principles  that  do  violence  to  our  fundamental  con- 
ceptions of  life  processes.  To  introduce  "  ferments  "  to  explain 
so  many  obscure  points  in  physiology,  as  the  conversion  of 
peptone  in  the  blood,  for  example,  is  taking  refuge  in  a  way 
that  does  no  credit  to  science. 

Without  denying  that  endosmosis,  etc.,  may  play  a  part  in 
the  vital  processes  we  are  considering,  we  believe  a  truer  view 
of  the  whole  matter  will  be  ultimately  reached.  In  the  mean 
time  we  think  it  best  to  express  our  belief  that  we  are  ignorant 
of  the  real  nature  of  absorption  in  great  part  ;  but  we  think 
that,  if  the  alimentary  tract  were  regarded  as  doing  for  the 
digested  food  (chyle,  etc.)  some  such  work  as  certain  other 
glands  do  for  the  blood,  we  would  be  on  the  way  to  a  truer  con- 
ception of  the  real  nature  of  the  processes. 

It  would  then  be  possible  to  understand  that  proteids,  either 
in  the  form  of  soluble  or  insoluble  substances,  including  pep- 
tone, might  be  taken  in  hand  and  converted  by  a  true  vital 
process  into  the  constituents  of  the  blood. 

If  we  were  to  regard  the  kidney  as  manufacturing  useful 
instead  of  harmful  products,  the  resemblance  in  behavior  would 
in  many  points  be  parallel.  We  have  seen  that  physical  expla- 
nations of  the  functions  of  the  kidney  have  failed,  and  that  it 
must  be  regarded  even  in  those  parts  that  eliminate  most  water 
as  a  genuine  secreting  mechanism. 

We  wish  to  present  a  somewhat  truer  conception  of  the 
lymph  that  is  separated  from  the  capillaries  and  bathes  the 
tissues. 

We  would  regard  its  separation  as  a  true  secretion,  and  not 
a  mere  diffusion  dependent  wholly  on  blood-pressure.  The 
mere  ligature  of  a  vein  does  not  suffice  to  cause  an  excess  of 
diffusion,  but  the  vaso-motor  nerves  have  been  shown  to  be 
concerned.  The  effusions  that  result  from  pathological  pro- 
cesses do  not  correspond  with  the  lymph — that  is,  the  nutrient 
material — provided  by  the  capillaries  for  the  tissues.  These 
vessels  are  more  than  mere  carriers ;  they  are  secretors — in  a 
sense  they  are  glands.  We  have  seen  that  in  the  foetus  they 
function  both  as  respiratory  and  nutrient  organs  in  the  allan- 
tois  and  yelk-sac,  and,  in  our  opinion,  they  never  wholly  lose 
this  function. 

The  kind  of  lymph  that  bathes  a  tissue,  we  believe,  depends 
on  its  nature  and  its  condition  at  the  time,  so  that,  as  we  view 


352  COMPARATIVE  PHYSIOLOGY. 

tissue-lymph,  it  is  not  a  mere  effusion  with  which  the  tissues, 
for  which  it  is  provided,  have  nothing-  to  do.  The  differences 
may  be  beyond  our  chemistry  to  determine,  but  to  assume  that 
all  lymph  poured  out  is  alike  is  too  crude  a  conception  to  meet 
the  facts  of  the  case.  Glands,  too,  it  will  be  remembered,  derive 
their  materials,  like  all  other  tissues,  not  directly  from  the 
blood,  but  from  the  lymph.  We  believe  that  the  cells  of  the 
capillaries,  like  all  others,  are  influenced  by  the  nervous  system, 
notwithstanding  that  nerves  have  not  been  traced  terminating 
in  them. 

It  is  to  be  borne  in  mind  that  the  lymph,  like  the  blood, 
receives  tissue  waste-products — in  fact,  it  is  very  important  to 
realize  that  the  lymph  is,  in  the  first  instance,  a  sort  of  better 
blood — an  improved,  selected  material,  so  far  as  any  tissue  is 
concerned,  which  becomes  gradually  deteriorated. 

We  have  not  the  space  to  give  all  the  reasons  on  which  the 
opinions  expressed  above  are  founded ;  but,  if  the  student  has 
become  imbued  with  the  principles  that  pervade  this  work  thus 
far,  he  will  be  prepared  for  the  attitude  we  have  taken,  and 
sympathize  with  our  departures  from  the  mechanical  (physical) 
physiology. 

We  think  it  would  be  a  great  gain  for  physiology  if  the  use 
of  the  term  "  absorption."  as  applied  to  the  alimentary  tract, 
were  given  up  altogether,  as  it  is  sure  to  lead  to  the  substitu- 
tion of  the  gross  conceptions  of  physical  processes  instead  of 
the  subtle  though  at  present  rather  indefinite  ideas  of  vital 
processes.  We  prefer  ignorance  to  narrow,  artificial,  and  er- 
roneous views. 

Pathological.— Under  certain  circumstances,  of  which  oue  is 
obstruction  to  the  venous  circulation  or  the  lymphatics,  fluid 
may  be  poured  out  or  effused  into  the  neighboring  tissues  or  the 
serous  cavities.  This  is  of  very  variable  composition,  but  always 
contains  enough  salts  and  proteids  to  remind  one  of  the  blood. 

•  Such  fluids  are  often  spoken  of  as  "lymph,"  though  the 
resemblance  to  normal  tissue-lymph  is  but  of  the  crudest  kind; 
and  the  condition  of  the  vessels  when  it  is  secreted,  if  such  a 
term  is  here  appropriate,  is  not  to  be  compared  to  the  natural 
separation  of  the  normal  lymph— in  fact,  were  this  not  so,  it 
would  be  identical  with  the  latter,  which  it  is  not.  When  such 
effusions  take  place  they  are  in  themselves  evidence  of  altered 
(and  not  merely  increased)  function. 

The  Faeces.— The  fseces  may  be  regarded  in  at  least  a  three- 


DIGESTION   OF   FOOD.  353 

fold  aspect.  They  contain  undigested  and  indigestible  rem- 
nants, the  ferments  and  certain  decomposition  products  of  the 
digestive  fluids,  and  true  excretory  matters. 

In  carnivorous  and  omnivorous  animals,  including  man, 
the  undigested  materials  are  those  that  have  escaped  the  action 
of  the  secretions — such  as  starch  and  fats — together  with  those 
substances  that  the  digestive  juices  are  powerless  to  attack, 
as  horny  matter,  hairs,  elastic  tissue,  etc. 

In  vegetable  feeders  a  larger  proportion  of  chlorophyl,  cel- 
lulose, and  starch  will,  of  course,  be  found. 

These,  naturally,  are  variable  with  the  individual,  the  spe- 
cies, and  the  vigor  of  the  digestive  organs  at  the  time. 

Besides  the  above,  certain  products  are  to  be  detected  in  the 
faeces  plainly  traceable  to  the  digestive  fluids,  and  showing 
that  they  have  tmdergone  chemical  decomposition  in  the  ali- 
mentary tract,  such  as  cholalic  acid,  altered  coloring-matters 
like  urobilin,  derivable  probably  from  bilirubin;  also  ckoles- 
terin,  fatty  acids,  insoluble  soaps  (calcium,  magnesium),  to- 
gether with  ferments,  having  the  properties  of  pepsin  and 
amylopsin.     Mucus  is  also  abundant  in  the  faeces. 

We  know  little  of  the  excretory  products  proper,  as  they 
probably  normally  exist  in  small  quantity,  and  it  is  not  impos- 
sible that  some  of  the  products  of  the  decomposition  of  the 
digestive  juices  may  be  reabsorbed  and  worked  over  or  excreted 
by  the  kidneys,  etc. 

There  is,  however,  a  recognized  non-nitrogenous  crystalline 
body  known  as  excretin,  which  contains  sulphur,  salts,  and 
pigments,  and  that  may  rank  perhaps  as  a  true  excretion  of 
the  intestine. 

It  is  well  known  that  bacteria  abound  in  the  alimentary 
tract,  though  their  number  is  dependent  on  a  variety  of  circum- 
stances, including  the  kind  of  food  and  the  condition  in  which 
it  is  eaten.  These  minute  organisms  feed,  of  course,  and  to  get 
their  food  produce  chemical  decompositions.  Skatol  and  indol 
are  possibly  thus  produced,  and  give  the  faecal  odor  to  the  con- 
tents of  the  intestine.  But  as  yet  our  ignorance  of  these 
matters  is  greater  than  our  knowledge — a  remark  which  ap- 
plies to  the  excretory  functions  of  the  alimentary  tract  gen- 
erally. 

Pathological.— The  facts  revealed  by  clinical  and  pathologi- 
cal study  leave  no  doubt  in  the  mind  that  the  intestine  at  all 
events  may,  when  other  glands,  like  the  kidney,  are  at  fault, 
23 


354  COMPARATIVE  PHYSIOLOGY. 

undertake  an  unusual  share  of  excretory  work,  probably  even 
to  the  length  of  discharging  urea. 

Obscure  as  the  subject  is,  and  long  as  it  may  be  before  we 
know  exactly  what  and  how  matter  is  thus  excreted,  we  think 
that  it  will  greatly  advance  us  toward  a  true  conception  of  the 
vital  processes  of  the  mammalian  body  if  we  regard  the  ali- 
mentary tract  as  a  collection  of  organs  with  both  a  secreting 
and  excreting  function ;  that  what  we  have  been  terming  ab- 
sorption is  in  the  main,  at  least,  essentially  secretion  or  an 
allied  process ;  and  that  the  parts  of  this  long  train  of  organs 
are  mutually  dependent  and  work  in  concert,  so  that  when  one 
is  lacking  in  vigor  or  resting  to  a  greater  or  less  degree,  the 
others  make  up  for  its  diminished  activity ;  and  that  the  whole 
must  work  in  harmony  with  the  various  excretory  organs,  as 
an  excretor  itself,  and  in  unison  with  the  general  state  of  the 
economy.  We  are  convinced  that  even  as  an  excretory  mech- 
anism one  part  may  act  (vicariously)  for  another. 

Of  course,  in  disease  the  condition  of  the  fasces  is  an  indica- 
tion of  the  state  of  the  digestive  organs ;  thus  color,  consistence, 
the  presence  of  food  in  lumps,  the  odor,  and  many  other  points 
tell  a  plain  story  of  work  left  undone,  ill-done,  or  disordered 
by  influences  operating  from  within  or  from  without  the  tract. 
The  intelligent  physician  acts  the  part  of  a  qualified  inspector, 
surveying  the  output  of  a  great  factory,  and  drawing  conclu- 
sions in  regard  to  the  kind  of  work  which  the  operatives  have 
performed. 

THE   CHANGES   PRODUCED  IN   THE  FOOD  IN   THE 
ALIMENTARY   CANAL. 

We  have  now  considered  the  method  of  secretion,  the  secre- 
tions themselves,  and  the  movements  of  the  various  parts  of 
the  digestive  tract,  so  that  a  brief  statement  of  the  results  of 
all  this  mechanism,  as  represented  by  changes  in  the  food,  will 
be  appropriate.  We  shall  assume  for  the  present  that  the  effects 
of  the  digestive  juices  are  substantially  the  same  in  the  body  as 
in  artificial  digestion. 

Among  mammals  food  is,  in  the  mouth,  comminuted  (except 
in  the  case  of  the  carnivora,  that  bolt  it  almost  whole,  and  the 
ruminants,  that  simply  swallow  it  to  be  regurgitated  for  fresh 
and  complete  mastication),  insalivated,  and,  in  most  species, 
chemically  changed,  but  only  in  so  far  as  starch  is  concerned. 


DIGESTION   OF  POOD. 


355 


Deglutition  is  the  result  of  the  co-ordinated  action  of  niany 
muscular  mechanisms,  and  is  reflex  in  nature.  The  oesophagus 
secretes  mucus,  which  lubricates  its  walls,  and  aids  mechan- 
ically in  the  transport  of  the  food  from  the  mouth  to  the  stom- 
ach. In  the  stomach,  by  the  action  of  the  gastric  juice,  food 
is  further  broken  up,  the  proteid  covering  of  fat-cells  is  digested, 
and  the  structure  of  muscle,  etc.,  disappears.  Proteid  matters 
become  peptone,  and  in  some  animals  fat  is  split  up  into  free 
fatty  acid  and  glycerin  ;  but  the  digestion  of  fat  in  the  stom- 
ach is  very  limited  at  best  and  probably  does  not  go  on  to 
emulsification  or  saponification.     The  digestion  of  starch  con- 


Fig.  288. — Matters  taken  from  pyloric  portion  of  stomach  of  dog  during  digestion  of 
mixed  food  (after  Bernard),  a.  disintegrated  muscular  fibers,  striae  having  disap- 
peared; b,  c,  muscular  fibers  in  which  stria;  have  partly  disappeared;  d,  d,  d,  glob- 
ules of  fat;  e,  e,  starch:  g,  molecular  granules. 

tinues  in  the  stomach  until  the  reaction  of  the  food-mass  be- 
comes acid.  This  in  the  hog  may  not  be  far  from  one  to  two 
hours,  and  the  amylolytic  ferment  acts  with  great  rapidity  even 
without  the  body.  The  food  is  moved  about  to  a  certain  ex- 
tent, so  as  to  expose  every  part  freely  to  the  mucous  mem- 
brane and  its  secretions.  It  is  likely  that  the  sugar  resulting 
from  the  digestion  of  starch,  the  peptones,  and,  to  some  ex- 
tent, the  fat  formed  (if  any),  is  received  into  the  blood  from 
the  stomach. 


356  '  COMPARATIVE  PHYSIOLOGY. 

As  the  partially  digested  mass  (chyme)  is  passed  on  into  the 
intestine  as  a  result  of  the  action  of  the  alkaline  hile,  the  para- 
peptone,  pepsin,  and  bile-salts  are  deposited.  Certain  of  the 
constituents  of  digestion  are  thus  delayed,  a  portion  of  the  pep- 
sin is  probably  absorbed,  either  altered  or  unaltered,  and  pep- 
sin is  thus  got  rid  of,  making  the  way  clear,  so  to  speak,  for  the 
action  of  trypsin.  At  all  events,  digestion  in  one  part  of  the 
tract  is  antagonized  by  digestion  in  another,  but  we  must  also 
add  supplemented. 

The  fat,  which  had  been  but  little  altered,  is  emulsified  by 
the  joint  action  of  the  bile  and  pancreatic  secretion ;  a  portion 
is  saponified,  which  again  helps  in  emulsification,  while  an  addi- 
tional part,  in  form  but  little  changed,  is  probably  dealt  with  by 
the  absorbents. 

Proteid  digestion  is  continued,  and,  besides  peptones,  nitro- 
genous crystalline  bodies  are  formed  (leucin  and  tyrosinj,  but 
under  what  conditions  or  to  what  extent  is  not  known;  though 
the  quantity  is  likely  very  variable,  both  with  the  species  of 
animal  and  the  circumstances,  such  as  quantity  and  quality  of 
food ;  and  it  is  likely  also  dependent  not  a  little  on  the  rate  of 
absorption.  It  seems  altogether  probable  that  in  those  that  use 
an  excess  of  nitrogenous  food  more  of  these  bodies  are  formed, 
and  thus  give  an  additional  work  to  the  excreting  organs,  in- 
cluding the  liver.  But  the  absence  of  albumin  from  healthy 
faeces  points  to  the  complete  digestion  of  proteids  in  the  ali- 
mentary canal.  Plainly  the  chief  work  of  intestinal  digestion 
is  begun  and  carried  on  in  the  upper  part  of  the  tract,  where 
the  ducts  of  the  main  glands  are  to  be  found. 

The  contents  of  the  intestine  swarm  with  bacteria,  though 
these  are  probably  kept  under  control,  to  some  extent,  by  the 
bile,  the  functions  of  which  as  an  antiseptic  we  have  already 
considered. 

The  removal  of  fats  by  the  villi  will  be  shortly  considered. 
The  other  products  of  digestion  probably  find  their  way  into 
the  general  circulation  by  the  portal  blood,  passing  through 
the  liver,  which  organ  modifies  some  of  them  in  ways  to  be 
examined  later. 

The  valvulce  conniventes  greatly  increase  the  surface  of  the 
intestine,  and  retard  the  movements  of  the  partially  digested 
mass,  both  of  which  are  favorable.  The  peristaltic  movements 
of  the  small  gut  serve  the  obvious  purpose  of  moving  on  the 
digesting  mass,  thus  making  way  for  fresh  additions  of  chyme 


DIGESTION   OP  FOOD.  357 

from  the  stomach,  and  carrying  on  the  more  elaborated  con- 
tents to  points  where  they  can  receive  fresh  attention,  both 
digestive  and  absorptive. 

Comparative. — -In  man,  the  carnivora,  and  some  other  groups, 
it  is  likely  that  digestion  in  the  large  intestine  is  slight,  the  work 
being  mostly  completed — at  all  events,  so  far  as  the  action  of 
the  secretions  is  concerned — before  this  division  of  the  tract  is 
reached,  though  doubtless  absorption  goes  on  there  also.  The 
muscular  strength  of  this  gut  is  important  in  the  act  of  defe- 
cation. 

But  the  great  size  of  the  large  intestine  in  ruminants — in 
the  horse,  etc. — together  with  the  bulky  character  of  the  food 
of  such  animals,  points  to  the  existence  of  possibly  extensive 
processes  of  which  we  are  ignorant.  It  is  generally  believed 
that  food  remains  but  a  short  time  in  the  stomach  of  the  horse, 
and  that  the  caecum  is  a  sort  of  reservoir  in  which  digestive 
processes  are  in  progress,  and  also  for  water. 

Fermentations  go  on  in  the  intestine,  and  probably  among 
ruminants  they  are  numerous  and  essential,  though  our  actual 
knowledge  of  the  subject  is  very  limited. 

The  gases  found  in  the  stomach  are  atmospheric  air  (swal- 
lowed) and  carbon  dioxide,  derived  from  the  blood.  Those  of 
the  intestine  are  nitrogen,  hydrogen,  carbonic  anhydride,  sul- 
phuretted hydrogen,  and  marsh-gas,  the  quantity  varying  con- 
siderably with  the  diet.  In  herbivora  the  quantity  of  C02  and 
CH4  is  large. 

Although  our  knowledge  of  the  actual  processes  by  which 
food  is  digested  in  the  domestic  animals  is  meager,  there  are 
certain  considerations  to  which  it  may  be  well  to  give  promi- 
nence at  this  point. 

The  whole  subject  becomes  clearer  and  the  way  is  paved  for 
more  exact  and  comprehensive  knowledge  if  it  be  borne  in 
mind  that  the  entire  alimentary  tract  has  a  common  embryo- 
logical  origin  from  the  splanchnopleure  (Fig.  225,  etc.),  consist- 
ing of  outer  mesoblast  and  lining  hypoblast,  the  former  giving 
rise  to  the  muscular  and  other  less  essential  structures,  the  lat- 
ter to  the  all-important  glandular  epithelium.  But  of  all  re- 
gions the  alimentary  tract  has  been  modified  in  relation  to 
the  development  and  habits  of  the  animal  group.  It  can  not 
be  too  well  remembered  that  digestion  is  highly  complex,  with 
one  organ  and  one  process  supplementary  to  another. 

If  mastication  is  imperfect,  as  in  the  camivora,  gastric  diges- 


358  COMPARATIVE   PHYSIOLOGY. 

tion  is  unusually  active,  as  is  well  seen  in  the  dog ;  if  the  stom- 
ach is  capacious  the  intestine  is  shorter,  also  exemplified  in 
this  group.  The  stomach  may  be  small  and  the  small  intes- 
tines not  lengthy,  but  the  large  intestine  of  enormous  size,  as  in 
the  horse. 

When  the  quantity  of  starchy  matters  found  in  the  food  of 
the  animal  is  large,  provision  is  made  for  its  digestion  in  sev- 
eral parts  of  the  alimentary  tract.  This  is  seen  in  the  horse 
and  other  herbivora.  Mastication  is  fairly  complete  in  these 
animals,  yet  a  part  of  the  small  stomach  of  the  horse  is  a  sort 
of  oesophageal  dilatation  (Fig.  266)  in  which  amylolytic  diges- 
tion goes  on  by  the  action  of  the  swallowed  saliva  and  possibly 
by  a  ferment  provided  in  this  region  of  the  organ. 

The  gastric  juice  of  the  horse  has  been  proved  capable  of 
digesting  starch,  possibly  because  mixed  with  the  swallowed 
saliva.  The  stomach  of  the  pig  is  large,  and  both  proteid  and 
starchy  digestion  exceedingly  active.  In  the  intestines  the  pro- 
cesses are  of  brief  duration,  but  very  effective. 

Digestion  in  the  upper  part  of  the  small  intestines  is,  in 
some  animals,  as  the  horse,  really  a  continuation  of  that  in  the 
stomach ;  or,  at  all  events,  the  contents  of  the  duodenum  and 
jejunum  are  usually  acid  in  reaction,  so  that  the  digestion 
peculiar  to  one  region  of  the  tract  does  not  always  abruptly 
end  when  food  has  left  that  part.  The  readiness  with  which 
food  passes  from  the  stomach  into  the  intestines  is  very  vari- 
able in  different  animals,  and  even  in  the  same  animal  under 
different  circumstances.  In  the  horse  the  pyloric  orifice  seems 
never  to  be  very  tightly  closed,  though  in  most  of  our  domestic 
animals  the  reverse  is  the  case ;  and  with  them  the  quantity  of 
undigested  material,  as  fat,  that  passes  into  the  small  intestine 
depends  on  the  rate  of  digestion  and  absorption  in  the  latter. 

In  the  horse,  if  water,  or  even  hay,  be  given  after  oats  a  por- 
tion of  the  latter  is  soon  carried  on  into  the  intestines,  so  that 
the  obvious  rule  for  feeding  such  an  animal  is  to  give  the  water 
and  hay  before  the  oats,  or,  at  least,  the  water  and  no  hay  im- 
mediately after  the  oats. 

Digestion  in  the  large  intestine  is  of  great  importance  in  the 
monogastric  herbivora,  as  the  horse.  The  caecum  is  of  enor- 
mous size — about  twice  that  of  the  stomach — and  has  communi- 
cation with  the  colon  by  a  small  opening,  so  that  it  furnishes  a 
sort  of  supplementary  reservoir  for  digestion  as  well  as  for 
water.     As  the  results  of  experiments,  it  has  been  concluded 


DIGESTION  OF  FOOD.  359 

that  food  is  found  in  the  stomach  twelve  hours  after  feeding ; 
in  the  caecum  after  twenty-four  hours,  with  a  residue  in  the 
jejunum ;  after  forty-eight  hours,  in  the  ventral  colon,  with  re- 
mains in  the  caecum;  after  seventy-two  hours,  in  the  dorsal 
colon ;  and  after  ninety  hours  in  the  dorsal  colon  and  rectum. 

The  caecum  appears  to  digest  large  quantities  of  cellulose, 
which  does  not  seem  to  be  affected  by  either  the  saliva,  gastric, 
or  pancreatic  juices.  The  pi'ocess  is  ill  understood.  In  her- 
bivora  the  large  intestine  takes  some  very  important  part — in 
digestion  and  absorption — and  we  would  again  remind  the  stu- 
dent that  the  latter  term  has  been  used  in  a  very  vague  if  not 
unwarrantable  sense.  It  is  important  for  the  practitioner  to 
bear  in  mind  that  nutrient  enemata  can  be  utilized  for  the  gen- 
eral good  of  the  economy  when  passed  into  either  the  large  or 
small  intestine. 

During  the  suckling  period  digestion  in  all  the  various 
groups  of  animals  is  probably  closely  analogous.  At  this  time, 
in  ruminants,  the  first  three  divisions  of  the  stomach  are  but 
slightly  developed. 

Pathological. — In  subjects  of  a  highly  neurotic  temperament 
and  unstable  nervous  system  it  sometimes  happens  that  im- 
mense quantities  of  gas  are  belched  from  an  empty  stomach  or 
distend  the  intestines. 

It  is  known  that  the  oxygen  swallowed  is  absorbed  into  the 
blood,  and  the  carbonic  anhydride  found  in  the  stomach  de- 
rived from  that  fluid. 

It  will  thus  be  seen  that  the  alimentary  tract  has  not  lost  its 
respiratory  functions  even  in  man,  and  that  these  may  in  cer- 
tain instances  be  inordinately  developed  (reversion). 

SPECIAL   CONSIDERATIONS. 

It  is  a  matter  well  recognized  by  those  of  much  experience 
in  breeding  and  keeping  animals  with  restricted  freedom  and 
under  other  conditions  differing  widely  from  the  natural  ones 
— i.  e.,  those  under  which  the  animals  exist  in  a  wild  state — that 
the  nature  of  the  food  must  vary  from  that  which  the  untamed 
ancestors  of  our  domestic  animals  used.  Food  may  often  with 
advantage  be  cooked  for  the  tame  and  confined  animal.  The 
digestive  and  the  assimilative  powers  have  varied  with  other 
changes  in  the  organism  brought  about  by  the  new  surround- 
ings.    So  much  is  this  the  case,  that  it  is  necessary  to  resort  to 


360  COMPARATIVE   PHYSIOLOGY. 

common  experience  and  to  more  exact  experiments  to  ascertain 
the  best  methods  of  feeding  animals  for  fattening,  for  work, 
or  for  breeding.  Inferences  drawn  from  the  feeding  habits  of 
wild  animals  allied  to  the  tame  to  be  valuable  must  always, 
before  being  applied  to  the  latter,  be  subjected  to  correction  by 
the  results  of  experience. 

It  is  now  well  established  by  experience  that  animals  kept 
in  confinement  must  have,  in  order  to  escape  disease  and  attain 
the  best  results  on  the  whole,  a  diet  which  not  only  imitates 
that  of  the  corresponding  wild  forms  generally,  but  even  in 
details,  with,  it  may  be,  altered  proportions  or  added  constitu- 
ents, in  consequence  of  the  difference  in  the  environment.  To 
illustrate:  poultry  can  not  be  kept  healthy  confined  in  a  shed 
without  sand,  gravel,  old  mortar,  or  some  similar  preparation ; 
and  for  the  best  results  they  must  have  green  food  also,  as 
lettuce,  cabbage,  chopped  green  clover,  grass,  etc.  They  must 
not  be  provided  with  as  much  food  as  if  they  had  the  exercise 
afforded  by  running  hither  and  thither  over  a  large  field.  We 
have  chosen  this  case  because  it  is  not  commonly  recognized 
that  our  domesticated  birds  have  been  so  modified  that  special 
study  must  be  made  of  the  environment  in  all  cases  if  they  are 
not  to  degenerate.  The  facts  in  regard  to  horned  cattle,  horses, 
and  dogs  are  perhaps  better  known. 

Cooking  greatly  alters  the  chemical  composition,  the  me- 
chanical condition,  and,  in  consequence,  the  flavor,  the  digesti- 
bility, and  the  nutritive  value  of  foods.  To  illustrate:  meat  in 
its  raw  condition  would  present  mechanical  difficulties,  the  di- 
gestive fluids  permeating  it  less  completely;  an  obstacle,  how- 
ever, of  far  greater  magnitude  in  the  case  of  most  vegetable 
foods.  By  cooking  certain  chemical  compounds  are  replaced 
by  others,  while  some  may  be  wholly  removed.  As  a  rule, 
boiling  is  not  a  good  form  of  preparing  meat,  because  it  with- 
draws not  only  salts  of  importance,  but  proteids  and  the  ex- 
tractives— nitrogenous  and  other.  Beef-tea  is  valuable  chiefly 
because  of  these  extractives,  though  it  also  contains  a  little 
gelatin,  albumin,  and  fats. 

Meat,  according  to  the  heat  employed,  may  be  so  cooked  as 
to  retain  the  greater  part  of  its  juices  within  it  or  the  reverse. 
With  a  high  temperature  (65°  to  70°  C.)  the  outside  in  roasting 
may  bo  so  quickly  hardened  as  to  retain  the  juices. 

In  feeding  dogs  it  is  both  physiological  and  economical  to 
give  the  animal  the  broth  as  well  as  the  meat  itself. 


DIGESTION   OF   FOOD.  361 

It  is  remarkable  in  the  highest  degree  that  man's  appetite, 
or  the  instinctive  choice  of  food,  has  proved  wiser  than  our 
science.  It  would  he  impossible  even  yet  to  match,  by  calcula- 
tions based  on  any  data  we  can  obtain,  a  diet  for  each  man  equal 
upon  the  whole  to  what  his  instincts  prompt.  With  the  lower 
mammals  we  can  prescribe  with  greater  success.  At  the  same 
time  chemical  and  physiological  science  can  lay  down  general 
principles  based  on  actual  experience,  which  may  serve  to  cor- 
rect some  artificialities  acquired  by  perseverance  in  habits  that 
were  not  based  on  the  true  instincts  of  a  sound  body  and  a 
healthy  mental  and  moral  nature;  for  the  influence  of  the 
latter  can  not  be  safely  ignored  even  in  such  discussions  as  the 
present.  These  remarks,  however,  are  meant  to  be  suggestive 
rather  than  exhaustive. 

We  may  with  advantage  inquire  into  the  nature  of  hunger 
and  thirst.  These,  as  we  know,  are  safe  guides  usually  in  eat- 
ing and  drinking. 

After  a  long  walk  on  a  warm  day  one  feels  thirsty,  the 
mouth  is  usually  dry;  at  all  events,  moistening  the  mouth, 
especially  the  back  of  it  (pharynx),  will  of  itself  partially  re- 
lieve thirst.  But  if  we  remain  quiet  for  a  little  time  the  thirst 
grows  less,  even  if  no  fluid  be  taken.  The  dryness  has  been 
relieved  by  the  natural  secretions.  If,  however,  fluid  be  intro- 
duced into  the  blood  either  directly  or  through  the  alimentary 
canal,  the  thirst  is  also  relieved  speedily.  The  fact  that  we 
know  when  to  stop  drinking  water  shows  of  itself  that  thei'e 
must  be  local  sensations  that  guide  us,  for  it  is  not  possible  to 
believe  that  the  whole  of  the  fluid  taken  can  at  once  have  en- 
tered the  blood, 

Hunger,  like  thirst,  may  be  mitigated  by  injections  into  the 
intestines  or  the  blood.  It  is,  therefore,  clear  that,  while  in  the 
case  of  hunger  and  thirst  there  is  a  local  expression  of  a  need, 
a  peculiar  sensation,  more  pronounced  in  certain  parts  (the 
fauces  in  the  case  of  thirst,  the  stomach  in  that  of  hunger), 
yet  these  may  be  appeased  from  within  through  the  medium 
of  the  blood,  as  well  as  from  without  by  the  contact  of  food  or 
water,  as  the  case  may  be. 

Up  to  the  present  we  have  assumed  that  the  changes 
wrought  in  the  food  in  the  alimentary  tract  were  identical  with 
those  produced  by  the  digestive  ferments  as  obtained  by  extracts 
of  the  organs  naturally  producing  them.  But  for  many  reasons 
it  seems  probable  that  artificial  digestion  can  not  be  regarded  as 


362  COMPARATIVE   PHYSIOLOGY. 

parallel  with  the  natural  processes  except  in  a  very  general 
way.  When  we  take  into  account  the  absence  of  muscular 
movements,  regulated  according  to  no  rigid  principles,  hut  vary- 
ing with  innumerable  circumstances  in  all  probability ;  the  ab- 
sence of  the  influence  of  the  nervous  system  determining  the 
variations  in  the  quantity  and  composition  of  the  outflow  of  the 
secretions;  the  changes  in  the  rate  of  so-called  absorption, 
which  doubtless  influences  also  the  act  of  the  secretion  of  the 
juices — by  these  and  a  host  of  other  considerations  we  are  led 
to  hesitate  before  we  commit  ourselves  too  unreservedly  to  the 
belief  that  the  processes  of  natural  digestion  can  be  exactly 
imitated  in  the  laboratory. 

What  is  it  which  enables  one  animal  to  digest  habitually 
what  may  be  almost  a  poison  to  another  ?  How  is  it  that  each 
one  can  dispose  readily  of  a  food  at  one  time  that  at  another  is 
quite  indigestible  ?  To  reply  that  in  the  one  case,  the  digestive 
fluids  are  poured  out  and  in  the  other  not,  is  to  go  little  below 
the  surface,  for  one  asks  the  reason  of  this,  if  it  be  a  fact,  as  it 
no  doubt  is.  When  we  look  further  into  the  peculiarities  of 
digestion,  etc.,  we  recognize  the  influence  of  race  as  such,  and 
in  the  race  and  the  individual  that  obtrusive  though  ill-under- 
stood fact — the  force  of  habit — operative  here  as  elsewhere. 
And  there  can  be  little  doubt  that  the  habits  of  animals,  as  to 
food  eaten  and  digestive  peculiarities  established,  become  or- 
ganized, fixed,  and  transmitted  to  posterity. 

It  is  probably  in  this  way  that,  in  the  course  of  the  evolu- 
tion of  the  various  groups  of  animals,  they  have  come  to  vary 
so  much  in  their  choice  of  diet  and  in  their  digestive  processes, 
did  we  but  know  them  thoroughly  as  they  are;  for  to  assume 
that  even  the  digestion  of  mammals  can  be  summed  up  in  the 
simple  way  now  prevalent  seems  to  us  too  broad  an  assump- 
tion.    The  field  is  very  wide,  and  as  yet  but  little  explored. 

The  law  of  rhythm  is  illustrated,  both  in  health  and  disease, 
in  striking  ways  in  the  digestive  tract.  An  animal  long  accus- 
tomed to  eat  at  a  certain  hour  of  the  day  will  experience  at  that 
time  not  only  hunger,  but  other  sensations,  probably  referable 
to  secretion  of  a  certain  quantity  of  the  digestive  juices  and  to 
the  movements  that  usually  accompany  the  presence  of  food  in 
the  alimentary  tract.  Hence  that  '*  colic  "  so  common  in  horses 
fed  at  irregular  times  and  unwisely,  after  excessive  work,  etc. 

It  is  well  known  that  defecation  at  periods  fixed,  even  within 
a  few  minutes,  has  become  an  established  habit  Avith  hosts  of 


DIGESTION   OP  FOOD.  363 

people;  and  the  same  is  to  a  degree  true  of  dogs,  etc.,  kept  in 
confinement,  that  are  taught  cleanly  habits,  and  encouraged 
therein  by  regular  attention  to  their  needs. 

This  tendency  (rhythm)  is  important  in  preserving  energy 
for  higher  ends,  for  such  is  the  result  of  the  operation  of  this 
law  everywhere. 

The  law  of  correlation,  or  mutual  dependence,  is  well 
illustrated  in  the  series  of  organs  composing  the  alimentary 
tract. 

The  condition  of  the  stomach  has  its  counterpart  in  the 
rest  of  the  tract  ;  thus,  when  St.  Martin  had  a  disordered 
stomach,  the  epithelium  of  his  tongue  showed  corresponding 
changes. 

We  have  already  referred  to  the  fact  that  one  part  may  do 
extra  work  to  make  up  for  the  deficiencies  in  another. 

It  is  confidently  asserted  of  late  that,  in  the  case  of  persons 
long  vmable  to  take  food  by  the  mouth,  nutritive  substances 
given  by  enemata  find  their  way  up  to  the  duodenum  by  anti- 
peristalsis.  Here,  then,  is  an  example  of  an  acquired  adaptive 
arrangement  under  the  stress  of  circumstances. 

It  can  not  be  too  much  impressed  on  the  mind  that  in  the 
complicated  body  of  the  mammal  the  work  of  any  one  organ 
is  constantly  varying  with  the  changes  elsewhere.  It  is  this 
mutual  dependence  and  adaptation — an  old  doctrine  too  much 
left  out  of  sight  in  modern  physiology — which  makes  the  at- 
tempt to  completely  un ravel  vital  processes  well-nigh  hopeless; 
though  each  accumulating  true  observation  gives  a  better  in- 
sight into  this  kaleidoscopic  mechanism. 

We  have  not  attempted  to  make  any  statements  as  to  the 
quantity  of  the  various  secretions  discharged.  This  is  large, 
doubtless,  but  much  is  probably  reabsorbed,  either  altered  or 
unaltered,  and  used  over  again.  In  the  case  of  fistula?,  the  con- 
ditions are  so  unnatural  that  any  conclusions  as  to  the  normal 
quantity  from  the  data  they  afford  must  be  highly  unsatisfac- 
tory. Moreover,  the  quantity  must  be  very  variable,  accord- 
ing to  the  law  we  are  now  considering.  It  is  well  known  that 
dry  food  provokes  a  more  abundant  discharge  of  saliva,  and 
this  is  doubtless  but  one  example  of  many  other  relations  be- 
tween the  character  of  the  food  and  the  quantity  of  secretion 
provided. 

Evolution.  —  We  have  from  time  to  time  either  distinctly 
pointed  out  or  hinted  at  the  evolutionary  implications  of  the 


364  COMPARATIVE   PHYSIOLOGY. 

facts  of  this  department  of  physiology.  The  structure  of  the 
digestive  organs,  plainly  indicating  a  rising  scale  of  complexity 
with  greater  and  greater  differentiation  of  function,  is,  beyond 
question,  an  evidence  of  evolution. 

The  law  of  natural  selection  and  the  law  of  adaptation, 
giving  rise  to  new  forms,  have  both  operated,  we  may  believe, 
from  what  can  be  observed  going  on  around  us  and  in  our- 
selves. The  occurrence  of  transitional  forms,  as  in  the  epi- 
thelium of  the  digestive  tract  of  the  frog,  is  also  in  harmony 
with  the  conception  of  a  progressive  evolution  of  structure  and 
function.  But  the  limits  of  space  will  not  permit  of  the  enu- 
meration of  details. 

Summary. — A  very  brief  resume  of  the  subject  of  digestion 
will  probably  suffice. 

Food  is  either  organic  or  inorganic  and  comprises  proteids, 
fats,  carbohydrates,  salts,  and  water  ;  and  each  of  these  must 
enter  into  the  diet  of  all  known  animals.  They  must  also  be 
in  a  form  that  is  digestible.  Digestion  is  the  reduction  of  food 
to  such  a  form  that  it  may  be  further  dealt  with  by  the  aliment- 
ary tract  prior  to  being  introduced  into  the  blood  (absorption). 
This  is  effected  in  different  parts  of  the  tract,  the  various  con- 
stituents of  food  being  differently  modified,  according  to  the 
secretions  there  provided,  etc.  The  digestive  juices  contain 
essentially  ferments  which  act  only  under  definite  conditions  of 
chemical  reaction,  temperature,  etc. 

The  changes  wrought  in  the  food  are  the  following :  starches 
are  converted  into  sugars,  proteids  into  peptones,  and  fats  into 
fatty  acids,  soaps,  and  emulsion ;  which  alterations  are  effected 
by  ptyalin  and  amylopsin,  pepsin  and  trypsin,  and  bile  and  pan- 
creatic steapsin,  respectively. 

Outside  the  mucous  membrane  containing  the  glands  are 
muscular  coats,  serving  to  bring  about  the  movements  of  the 
food  along  the  digestive  tract  and  to  expel  the  faeces,  the  circu- 
lar fibers  being  the  more  important.  These  movements  and  the 
processes  of  secretion  and  so-called  absorption  are  under  the 
control  of  the  nervous  system. 

The  preparation  of  the  digestive  secretions  involves  a  series 
of  changes  in  the  epithelial  cells  concerned,  which  can  be  dis- 
tinctly traced,  and  take  place  in  response  to  nervous  stimula- 
tion. 

These  we  regard  as  inseparably  bound  up  with  the  healthy 
life  of  the  cell.     To  be  natural,  it  must  secrete. 


DIGESTION  OP  FOOD.  365 

The  blood-vessels  of  the  stomach  and  intestine  and  the  villi 
of  the  latter  receive  the  digested  food  for  further  elaboration 
(absorption).  The  undigested  remnant  of  food  and  the  excre- 
tions of  the  intestine  make  up  the  faeces,  the  latter  being  ex- 
pelled by  a  series  of  co-ordinated  muscular  movements  essen- 
tially reflex  in  origin. 


THE   RESPIRATORY   SYSTEM. 


In  the  mammal  the  breathing1  organs  are  lodged  in  a  closed 
cavity,  separated  by  a  muscular  partition  from  that  in  which 
the  digestive  and  certain  other  organs  are  contained.  This 
thoracic  chamber  may  be  said  to  be  reserved  for  circulatory 
and  respiratory  organs  which,  we  again  point  out,  are  so  related 
that  they  really  form  parts  of  one  system. 

The  mammal's  blood  requires  so  much  aeration  (ventilation) 
that  the  lungs  are  very  large  and  the  respiratory  system  has 
become  greatly  specialized.  We  no  longer  find  the  skin  or  ali- 
mentary canal  taking  any  large  share  in  the  process;  and  the 
lungs  and  the  mechanisms  by  which  they  are  made  to  move  the 
gases  with  which  the  blood  and  tissues  are  concerned  become 
very  complicated. 

Our  studies  of  muscle  physiology  should  have  made  clear 
the  fact  that  tissue-life  implies  the  constant  consumption  of 
oxygen  and  discharge  of  carbonic  anhydride,  and  that  the  pro- 
cesses which  give  rise  to  this  are  going  on  at  a  rapid  rate ;  so 
that  the  demands  of  the  animal  for  oxygen  constantly  may  be 
readily  understood  if  one  assumes,  what  can  be  shown,  though 
less  readily  than  in  the  case  of  muscle,  that  all  the  tissues  are 
constantly  craving,  as  it  were,  for  this  essential  oxygen — well 
called  '"vital  air." 

Respiration  may,  then,  be  regarded  from  a  physical  and 
chemical  point  of  view,  though  in  this  as  in  other  instances  we 
must  be  on  our  guard  against  regarding  physiological  processes 
as  ever  purely  physical  or  purely  chemical.  The  respiratory 
process  in  the  mammal,  unlike  the  frog,  consists  of  an  active 
and  a  (largely)  passive  phase.  The  air  is  not  pumped  into  the 
lungs,  but  sucked  in.  So  great  is  the  complexity  of  the  lungs 
in  the  mammal,  that  the  frog's  lung  (which  may  be  readily 
understood  by  blowing  it  up  by  inserting  a  small  pipe  in  the 
glottic  opening  of  the  animal  and  then  ligaturing  the  distended 


TPIE   RESPIRATORY   SYSTEM. 


367 


organ)  may  be  compared  to  a  single  infundibulum  of  the  mam- 
malian lung. 

Assuming  that  the  student  is  somewhat  conversant  with  the 
coarse  and  fine  anatomy  of  the  respiratory  organs,  we  call  at- 


Fi<i.  2S'J.—  Lungs,  anterior  view  (Sappey ).  1,  upper  lobe  of  left  lung;  2,  lower  lobe;  3. 
fissure;  4,  notch  corresponding  to  apex  of  heart;  5.  pericardium;  (i.  upper  lobe  of 
right  lung;  7,  middle  lobe;  8,  lower  lobe;  9.  fissure:  10,  fissure;  11,  diaphragm; 
12.  anterior  mediastinum;  13.  thyroid  gland;  14.  middle  cervical  aponeurosis;"  15. 
process  of  attachment  of  mediastinum  to  pericardium;  1G,  10,  seventh  ribs;  17, 17. 
transversales  muscles;  18.  linea  alba. 


tention  to  the  physiological   aspects  of  some  points  in  their 
structure.     The  lungs  represent  a  membranous  expansion  of 


368 


COMPARATIVE   PHYSIOLOGY. 


great  extent,  lined  with  flattened  cells  and  supporting  innu- 
merable capillary  blood-vessels.     The  air  is  admitted  to  the  com- 


Fig.  290. — Bronchia  and  lungs,  posterior  view  (Sappey).  1,  1,  summit  of  lungs;  2,2, 
base  of  lungs;  3,  trachea;  4,  right  bronchus;  5,  division  to  upper  lobe  of  lung;  6, 
division  to  Tower  lobe;  7,  left  bronchus;  8,  division  to  upper  lobe;  9,  division  to 
lower  lobe;  10,  left  branch  of  pulmonary  artery;  11,  right  branch;  12,  left  auricle 
of  heart;  13,  left  superior  pulmonary  vein;  14,  left  inferior  pulmonary  vein;  15, 
right  superior  pulmonary  vein;  16,  right  inferior  pulmonary  vein;  17,  inferior  vena 
cava;  18,  left  ventricle  of  heart;  19,  right  ventricle. 

plicated  foldings  of  this  membrane  by  tubes  'which  remain, 
throughout  the  greater  part  of  their  extent,  open,  being  com- 
posed of  cartilaginous  rings,  completed  by  soft  tissues,  of  which 
plain  muscle-cells  form  an  important  part,  serving  to  main- 
tain a  tonic  resistance  against  pulmonary  and  bronchial  press- 
ure, as  well  as  serving  to  aid  in  the  act  of  coughing,  etc., 
so  important  in  expelling  foreign  bodies  or  preventing  their 
ingress. 

The  bronchial  tubes  are  lined  with  a  mucous  membrane, 
kept  moist  by  the  secretions  of  its  glands,  and  covered  with 
ciliated  epithelium,  as  are  also  the  nasal  passages,  which,  by 
the  outward  currents  they  create,  favor  diffusion  of  gases  and 
removal  of  excess  of  mucus.     The  thoracic  walls  and  the  lun^s 


THE   RESPIRATORY   SYSTEM.  369 

themselves  are  covered  with  a  tough  but  thin  membrane  lined 
with  flattened  cells,  which  secrete  a  small  quantity  of  fluid 
that  serves  to  maintain  the  surrounding  parts  in  a  moist  con- 


Fig.  291. — Mold  of  a  terminal  bronchus  and  a  group  of  air-cells  moderately  distended 
by  injection,  from  the  human  subject  (Robin). 

dition.  thus  lessening  friction.  The  importance  of  this  ar- 
rangement is  well  seen  when,  in  consequence  of  inflammation 
of  this  pleura,  it  becomes  diy,  giving  rise  during  each  respira- 
tory movement  to  a  friction-sound  and  a  painful  sensation. 
It  will  not  be  forgotten  that  this  membrane  extends  over  the 
diaphragm,  and  that,  in  consequence  of  the  lungs  completely 
filling  all  the  space  (not  occupied  by  other  organs)  during  every 
position  of  the  chest-walls,  the  costal  and  pulmonary  pleural 
surfaces  are  in  constant  contact.  By  far  the  greater  part  of 
the  lung-substance  consists  of  elastic  tissue,  thus  adapting  the 
principal  respiratory  organs  to  that  amount  of  distention  and 
recoil  to  which  they  are  ceaselessly  subjected  during  the  entire 
lifetime  of  the  animal. 
24 


370 


COMPARATIVE   PHYSIOLOGY. 


Fig.  292.— Section  of  the  parenchyma  of  the  human  lung,  injecterl  through  the  pul- 
monary artery  (Schulze).  a,  a,  a,  c,  c,  c,  walls  of  the  air-cells;  b,  small  arterial 
branch. 


THE   ENTRANCE   AND   EXIT    OF   AIR. 

Since  the  lungs  fill  up  so  completely  the  thoracic  cavity, 
manifestly  any  change  in  the  size  of  the  latter  must  lead  to 
an  increase  or  diminution  in  the  quantity  of  air  they  contain. 
Since  the  air  within  the  respiratory  organs  is  being  constantly 
robbed  of  its  oxygen,  and  rendered  impure  by  the  addition  of 
carbonic  dioxide,  the  former  must  be  renewed  and  the  latter 
expelled ;  and,  as  mere  diffusion  takes  place  too  slowly  to  ac- 
complish this  in  the  mammal,  this  process  is  assisted  by  the 
nervous  system  setting  certain  muscles  at  work  to  alter  the  size 
of  the  chest  cavity.  Because  of  the  ribs  being  placed  oblicpiely, 
it  follows  that  their  elevation  will  result  in  the  enlargement  of 
the  thoracic  cavity  in  the  antero-posterior  diameter ;  and,  as  the 
chest,  in  consequence,  gets  wider  from  above  downward,  also  in 
the  transverse  diameter;  which  is  moreover  assisted  by  the  ever- 
sion  of  the  lower  borders  of  the  ribs;  and,  if  the  convexity  of  the 
diaphragm  were  diminished  by  its  contraction  and  consequent 
descent,  it  would  follow  that  the  chest  would  be  increased  in 


THE   RESPIRATORY   SYSTEM. 


871 


the  vertical  diameter  also.  All  these  events,  favorable  to 
the  entrance  of  air,  actually  take  place  through  agencies  we 
must  now  consider.  The  student  is  recommended  to  look  into 
the  insertion,  etc.,  of  the  muscles  concerned,  to  which  we  can 
only  briefly  refer.  We  have  made  the  descriptions  and  cuts 
applicable  to  man,  so  that  it  may  be  easy  for  the  student  to  ver- 
ify all  essential  points  on  his  own  person.  Respiration  in  our 
domestic  animals  is  in  the  main  as  in  man. 

The  act  of  inspiration  commences  by  the   fixation  of  the 
uppermost  ribs,  beginning  with  the  first  two,  by  means  of  the 


Fig.  293. — Diagram  illustrating  elevation  of  ribs  in  inspiration  (B6clard).  The  dark 
lines  represent  the  ribs,  sternum,  and  costal  cartilages  in  inspiration. 

Fig.  294.— Diagrammatic  representation  of  action  of  diaphragm  in  respiration  (Her- 
mann). Vertical  section  throngh  second  rib  on  right  side.  The  broken  and  dot- 
ted lines  show  the  amount  of  the  descent  of  the  diaphragm  in  ordinary  and  in 
deep  inspiration. 


scaleni  muscles,  this  act  being  followed  up  by  the  contraction  of 
the  external  intercostals,  leading  to  the  elevation  of  the  other 
ribs  ;  at  the  same  time,  the  arch  of  the  diaphragm  descends  in 
consequence  of  the  contraction  of  its  various  muscular  bundles. 
Under  these  circumstances,  the  air  from  without  must  rush  in, 
or  a  vacuum  be  formed  in  the  thoracic  cavity  ;  and,  since  there 


372 


COMPARATIVE  PHYSIOLOGY, 


is  free  access  for  the  air  through  the  glottic  opening,  the  lungs 
are  of  necessity  expanded.  This  ingoing  air  has  had  to  over- 
come the  elastic  resistance  of  the  lungs,  which  amounts  to  about 


Fig.  2f  5.— Apparatus  to  illustrate  relations  of  intra-thoracic  and  external  pressures 
(after  Beaunis).  A  glass  bell-jar  is  provided  with  a  light  stopper,  through  which 
passes  a  branching  glass  tube  fitted  with  a  pair  of  elastic  bags  representing  lungs. 
The  bottom  of  the  jar  is  closed  by  rubber  membrane  representing  diaphragm.  A 
mercury  manometer  indicates  the  difference  in  pressure  within  and  without  the 
bell-jar.  In  left-hand  figure  it  will  be  seen  that  these  pressures  are  equal;  in  right 
(inspiration),  the  external  pressure  is  considerably  greater.  At  one  part  (6)  an 
elastic  membrane  fills  a  hole  in  jar,  representing  an  intercostal  space. 

five  millimetres  of  mercury  in  man,  as  ascertained  by  tying  a 
manometer  in  the  windpipe  of  a  dead  subject,  and  then  opening 
the  thorax  to  equalize  the  inside  and  outside  pressures,  when 

the  lungs  at  once  collapse  and 
the  manometer  shows  a  rise  of 
the  mercury  to  the  extent  indi- 
cated above.  To  this  we  must 
add  the  influence  of  the  tonic 
contraction  of  the  bronchial 
muscles  before  referred  to, 
though  this  is  probably  not  very 
great. 

That  there  are  variations  of 
intrapulmonary  pressure  may 
be  ascertained  by  connecting  a 
manometer  with  one  nostril — 
the  other  being  closed — or  with 
the  windpipe.  The  mercury 
shows  a  negative  pressure  with 
each  inspiratory,  and  a  positive 


Fig.  2%.— Dorsal  view  of  four  vertebras 
and  three  attached  ribs,  showing  at- 
tachment of  elevator  muscles  of  ribs 
and  intercostal*  (after  Allen  Thom- 
son). 1,  long  and  short  elevators;  2, 
external  intercostal;  3,  internal  in- 
tercostal. 


THE    RESPIRATORY   SYSTEM. 


373 


with  each  expiratory  act.  This  may  amount  to  from  30  to  70 
millimetres  with  strong  inspiration,  and  60  to  100  in  forcible  ex- 
piration. 

When  inspiration  ceases,  the  elastic  recoil  of  the  rib  carti- 
lages and  the  ribs  themselves,  and  of  the  sternum,  the  weight 
of  these  parts  and  that  of  the  attached  muscles,  etc.,  assists  in 
the  return  of  the  chest  to  its  original  position,  entirely  inde- 
pendently of  the  action  of  muscles.  Moreover,  with  the  de- 
scent of  the  diaphragm  the  abdominal  viscera  have  been  thrust 
down  and  compressed  together  with  their  included  gases;  when 
this  muscle  relaxes,  they  naturally  exert  an  upward  pressure. 
Putting  these  events  together,  it  is  not  difficult  to  understand 
why  the  air  should  be  squeezed  out  of  the  lungs,  the  elasticity 
of  which  latter  is.  as  wre  have  shown,  an  important  factor  in 
itself. 

The  Muscles  of  Respiration.— The  diaphragm  may  be  con- 
sidered the  most  important  single  respiratory  muscle,  and  can 
of  itself  maintain  respiration.  The 
scaleni  are  important  as  fixators 
of  the  ribs  ;  the  levatores  costa- 
rum  and  external  intercostals,  as 
normal  elevators.  The  quadra- 
tics lumborum  assists  the  dia- 
phragm by  fixing  the  last  rib. 
These,  with  the  serratus  posticus 
superior,  may  be  regarded  as  the 
principal  muscles  called  into  ac- 
tion in  an  ordinary  inspiration. 
The  muscles  used  in  an  ordinary 
expiratory  act  are  the  internal  in- 
tercostals, the  triungularis  sterni, 
and  serratus  posticus  inferior. 
In  forced  inspiration  the  lower 
ribs  are  drawn  down  and  re- 
tracted, giving  support  in  their 
fixed  position  to  the  diaphragm. 
The  scaleni,  pectorales,  serratus 
magnus,  latissimus  dorsi,  and  oth- 
ers are  called  into  action  ;  but. 
when  dyspnoea  becomes  extreme, 

as  in  one  with  a  fit  of  asthma,  nearly  all  the  muscles  of  the 
body  may  be  called  into  play,  even  the  muscles  of  the  face. 


Fig.  297.—  Laryngoscopy  views  of 
the  glottis,  etc.  (after  Quail)  and 
Czermak).  I.  Larynx  in  quiet 
breathing.  II.  During  a  deep  in- 
spiration.  In  this  case  the  rinsrs 
of  the  trachea  and  commence- 
ment of  bronchi  are  visible. 
Such  a  condition  is  persistent  iu 
many  forms  of  disease  in  which 
respiration  is  attended  with  dif- 
ficulty. 


374 


COMPARATIVE   PHYSIOLOGY. 


which  are  not  normally  active  at  all  or  but  very  slightly  in  nat- 
ural breathing. 

Facial  and  laryngeal  respiration  is  best  seen  in  such  ani- 
mals as  the  rabbit,  and  it  is  this  condition  which  is  ap- 
proximated in  disordered  states  in  man — in  fact,  when  from 
any  cause    inspiration    is   very   labored   (asthma,    diphtheria, 

etc.). 

In  man  and  most  mammals,  unlike  the  frog,  the  glottic 
opening  is  never  entirely  closed  during  any  part  of  the  respira- 
tory act,   though  it  undergoes  a  rhythmical   change  of  size, 


Fig.  2i>8. 


Fig.  299. 


Fig.  298. — Vertical  transverse  section  of  fresh-water  mussel  (Anodon)  through  heart 
(after  Huxley).  V.  ventricle;  a,  auricles;  r,  rectum;  p,  pericardium;  i.  inner,  o, 
outer  gill;  o',  vestibule  of  organ  of  Bojanus,  fi;  f.  foot;  m.tn,  mantle  lobes. 

Fig.  299.— Gill  of  fish  (perch),  to  illustrate  relations  of  different  blood-vessels,  etc., 
concerned  in  respiration  (after  Bell).  A,  branchial  artery;  B,  branchial  arch 
seen  in  cross-section;  V.  branchial  vein;  a,  V,  branches  of  artery  and  vein  re- 
spectively. 

widening  during  inspiration  and  narrowing  during  expiration, 
in  accordance  with  the  action  of  the  muscles  attached  to  the 
arytenoid  cartilages,  the  action  of  which  may  be  studied  in  man 
by  means  of  the  laryngscope. 

The  abdominal  muscles  have  a  powerful  rhythmical  action 
during  forced  respiration,  though  whether  they  function  dur- 


THE   RESPIRATORY   SYSTEM, 


375 


h\g  ordinary  quiet  breathing  is  undetermined  ;  if  at  all,  prob- 
ably but  slightly.  Though  the  removal  of  the  external  inter- 
costals  in  the  dog  and  some  other  animals  reveals  the  fact 
that  the  internal  intercostals  contract  alternately  with  the  dia- 


IV         V       VI  VII VIII  IX  X  XL 


XJJ    f         XII) 


XIV 


Fig.  300. — Diagram  of  scorpion,  most  of  the  appendages  having  been  removed  (after 
Huxley),  a,  mouth;  b,  alimentary  tract;  c,  anus;  d,  heart;  e.  pulmonary  sac:  /, 
position  of  ventral  ganglionated  cord;  q,  cerebral  ganglia;  T,  telson.  VII — XX, 
seventh  to  twentieth  somite.  IV,  V,  VI,  basal  joints  of  pedipalpi  and  two  fol- 
lowing pairs  of  limbs. 

phragm,  it  must  not  be  regarded  as  absolutely  certain  that  such 
is  their  action  when  their  companion  muscles  are  present,  for 
Nature  has  more  ways  than  one  of  accomplishing  the  same 
purpose — a  fact  that  seems  often  to  be  forgotten  in  reasoning 
from  experiments.  This  result,  however,  carries  some  weight 
with  it. 

Types  of  Respiration. — There  are  among  mammals  two  prin- 
cipal types  of  breathing  recognizable — the  costal  (thoracic)  and 
abdominal — according  as  the  movements  of  the  chest  or  the 
abdomen  (diaphragm)  are  the  more  pronounced. 

Personal  Observation.— The  student  would  do  well  at  this 
stage  to  test  the  statements  we  have  made  in  regard  to  the  respira- 
tory movements  on  the  human  subject  especially.  This  he  can 
very  well  do  in  his  own  person  when  stripped  to  the  waist  be- 
fore a  mirror.  Many  of  the  abnormalities  of  the  forced  respira- 
ation  of  disease  may  be  imitated — in  fact,  this  is  one  of  the 
departments  of  physiology  in  wThich  the  human  aspects  may  be 


376 


COMPARATIVE   PHYSIOLOGY, 


examined  into  by  a  species  of  experiment  on  one's  self  that  is 
as  simple  as  it  is  valuable. 

Comparative. — It  is  hoped  that  the  various  figures  accompa- 
nied by  descriptions,  introduced  in  this  and  other  chapters,  will 


#'&■ 


Fig.  303. 


Fig.  301. 


Fig.  301.— A.   Pulmonary  sac.     B.   Respiratory  leaflets  of  Scorpio  occitanus  (after 

Blancharch. 
Fig.  302.— Left  pulmonary  sac.  viewed  from  dorsal  aspect,  of  a  spider  (after  Duges). 

Pm.  pulmonary  lamella?;  Stg.  stigma,  or  opening  to  former. 

make  the  relations  of  the  circulation  and  respiration  in  the  va- 
rious classes  of  animals,  whether  terrestrial  or  aquatic,  evident 


Fig.  303  -A.  B.  Tadpoles  with  external  branchiae  (after  Huxley),  n.  nasai  sacs;  a, 
eye;  o,  ear:  /.'.  '>.  branchiae;  m,  mouth;  z.  horny  jaws;  s,  suckers;  d,  opercular 
(or  gill)  fold.  C.  More  advanced  frosts  larva,  y.  rudiment  of  hind-limb;  k.  s, 
single  branchial  aperture.  Owing  to  figure  not  having  been  reversed,  this  aper- 
ture seems  to  lie  on  right  instead  of  left  side. 

without  extended  treatment  of  the  subject  in  the  text.  What 
we  are  desirous  of  impressing  is  that  throughout  the  entire 
animal  kingdom  respiration  is  essentially  the  same  process:  that 


THE   RESPIRATORY   SYSTEM. 


377 


finally  it  resolves  itself  into  tissue-breathing — the  appropriation 
of  oxygen  and  the  excretion  of  carbon  dioxide.  Since  the  man- 
ner in  which  oxy- 
gen is  intro 
into  the  lungs  and 
foul  gases  expelled 
from  them  in  some 
reptiles  and  amphib- 
ians is  largely  dif- 
ferent from  the 
method  of  respira- 
tion in  the  mam- 
mal, we  call  atten- 
tion to  this  process 
in  an  animal  readily 
watched — the  com- 
mon frog.  This 
creature,  by  depress- 
ing the  floor  of  the 
mouth,  enlarges  his 
air-space  in  this  re- 
gion and  conse- 
quently the  air  free- 
ly enters  through 
the  nostrils ;  where- 
upon the  latter  are 
closed  by  a  sort  of 
valve,  the  glottis 
opened  and  the  air 
forced  into  the  lungs 
by  the  elevation  of 
the  floor  of  the 
mouth.  By  a  series 
of  flank  movements 
the  elasticity  of  the 
lungs  is  aided  in 
expelling  the  air 
through  the  now 
open  nostrils.  The 
inspiration  of  the 
turtle  and  some  oth- 
er reptiles  is  somewhat  similar 


Fir,.  304.— General  view  of  air-reservoirs  of  duck,  opened 
interiorly,  also  their  relations  with  principal  viscera 
of  trunk  (after  Sappey).  1,  1,  anterior  extremity  of 
cervical  reservoirs  ;  2,  thoracic  reservoir;  3,  anterior 
diaphragmatic  reservoir;  4,  posterior  ditto;  5,  abdom- 
inal reservoir;  a,  membrane  forming  anterior  dia- 
phragmatic reservoir;  b,  membrane  forming  posterior 
ditto;  c,  section  of  thoracoabdominal  diaphragm:  d, 
subpectoral  prolongation  of  thoracic  reservoir;  ■. 
pericardium;  f,  liver;  tj.  gizzard:  //.  intestines;  in, 
lieart;  n,  n,  section  of  great  pectoral  muscle  above  its 
insertion  into  the  humerus;  o.  anterior  clavicle;  p,  pos- 
terior clavicle  of  right  side  cut  and  turned  outward. 


In  the  case  of  aquatic  animals, 


378  COMPARATIVE   PHYSIOLOGY. 

both  invertebrate  and  vertebrate,  excepting  mammals,  the  blood 
is  freely  exposed  in  the  gills  to  oxygen  dissolved  in  the  water  as 
it  is  to  the  same  gas  mixed  with  nitrogen  in  terrestrial  animals. 
In  the  land-snail,  land-crab,  etc.,  we  have  a  sort  of  intermediate 
condition,  the  gills  being  kept  moist.  It  is  not  to  be  forgotten, 
however,  that  normally  the  respiratory  tract  of  mammals  is 
never  other  than  slightly  moist. 

THE   QUANTITY   OF  AIR  RESPIRED. 

We  distinguish  between  the  quantity  of  air  that  usually  is 
moved  by  the  thorax  and  that  which  may  be  respired  under 
special  effort,  which,  of  course,  can  never  exceed  the  capacity 
of  the  respiratory  organs. 

Accordingly,  we  recognize:  1.  Tidal  air,  or  that  which 
passes  in  and  out  of  the  respiratory  passages  in  ordinary  quiet 
breathing,  amounting  to  about  500  cc,  or  thirty  cubic  inches. 
2.  Complemental  air,  which  may  be  voluntarily  inhaled  by  a 
forced  inspiration  in  addition  to  the  tidal  air,  amounting  to 
1,500  cc,  or  about  100  cubic  inches.  3.  Supplemental  {reserve) 
air,  which  may  be  expelled  at  the  end  of  a  normal  respiration 
— i.  e.,  after  the  expulsion  of  the  tidal  air,  and  which  represents 
the  quantity  usually  left  in  the  lungs  after  a  normal  quiet  ex- 
piration, amounting  to  1,500  cc.  4.  Residual  air,  which  can 
not  be  voluntarily  expelled  at  all,  amounting  to  about  2,000  cc, 
or  120  cubic  inches.  Although  these  quantities  have  been  esti- 
mated for  man,  probably  a  similar  relation  (proportion)  between 
them  holds  for  the  domestic  animals. 

The  vital  capacity  is  estimated  by  the  quantity  of  air  that 
may  be  expired  after  the  most  forcible  inspiration.  This  will, 
of  course,  vary  with  the  age,  which  determines  largely  the  elas- 
ticity of  the  thorax,  together  with  sex,  position,  height,  and  a 
variety  of  other  circumstances.  But,  inasmuch  as  the  result 
may  be  greatly  modified  by  practice,  like  the  power  to  expand 
the  chest,  the  vital  capacity  is  not  so  valuable  an  indication  as 
might  at  first  be  supposed. 

It  is  important  to  bear  in  mind  that  the  tidal  air  is  scarcely 
more  than  sufficient  to  fill  the  upper  air-passages  and  larger 
bronchi,  so  that  it  requires  from  five  to  ten  respirations  to  re- 
move a  quantity  of  air  inspired  by  an  ordinary  act.  Very  much 
must,  therefore,  depend  on  diffusion,  the  quantity  of  air  remain- 
ing in  the  lungs  after  each  breath  being  the  sum  of  the  residual 


THE   RESPIRATORY  SYSTEM.  379 

and  reserve  air,  or  about  3,500  cc.  (220  cubic  iucbes).  Consider- 
ing the  creeping  slowness  of  the  capillary  circulation,  it  would 
not  be  supposed  that  the  respiratory  process  in  its  essential 
parts  should  be  the  rapid  one  that  a  greater  movement  of  the 
air  would  imply. 

THE   RESPIRATORY   RHYTHM. 

In  man,  and  most  of  our  domestic  mammals,  a  definite 
though  somewhat  different  relation  between  the  cardiac  and 
respiratory  movements  obtains,  there  being  about  three  to  five 
heart-beats  to  one  respiration,  which  would  make  the  rate  of 
breathing  in  man  about  sixteen  to  eighteen  per  minute.  Usual- 
ly, of  course,  the  largest  animals  have  the  slower  pulse  and  res- 
piration ;  and  this  is  an  invariable  rule  for  the  varieties  of  a 
species,  as  observable  in  the  canine  race,  to  mention  a  well- 
known  instance.  The  horse  breathes  9  to  12  times  in  a  minute; 
the  ox  15  to  20  ;  the  sheep  13  to  17  ;  and  the  dog  15  to  20. 

The  rate  of  the  respiratory  movements  is  to  some  extent  a 
measure  of  the  rapidity  of  the  oxidative  processes  in  the  body, 
as  witness  the  slow  and  intermittent  breathing  of  cold-blooded 
animals  as  compared  with  the  more  rapid  respiration  of  birds 
and  mammals  (Fig.  305). 

Pathological. — Any  condition  that  lessens  the  amount  of  re- 
spiratory surface,  or  diminishes  the  mobility  of  the  chest-walls, 
is  usually  accompanied  by  accelerated  movements,  but  beneath 
this  is  the  demand  for  oxygen,  part  of  the  avenues  by  which 
this  gas  usually  enters  having  been  closed  or  obstructed  by  the 
disease.  So  that  it  is  not  surprising  that,  in  consequence  of 
the  effusion  of  fluid  into  the  thoracic  cavity,  leading  to  the 
compression  of  the  lung,  the  opposite  one  should  be  called  into 
more  frequent  use,  and  even  enlarge  to  meet  the  demand. 
These  facts  show  how  urgent  is  the  need  for  constant  ventila- 
tion of  the  blood,  and  at  the  same  time  how  great  is  the  power 
of  adaptation  to  meet  the  emergency. 

The  difference  between  the  inspiratory  and  the  expiratory 
rhythm  may  be  gathered  by  watching  the  movements  of  the 
bared  chest,  or  more  accurately  from  a  graphic  record.  It  is 
usually  considered  that  expiration  is  only  slightly  longer  than 
inspiration,  and  that  any  marked  deviation  from  this  relation 
should  arouse  suspicion  of  disease.  Normally  the  respiratory 
pause  is  very  slight,  so  that  inspiration  seems  to  follow  directly 


380 


COMPARATIVE   PHYSIOLOGY. 


on  expiration  ;    though  the  latter  act  reminds  us  of  the  pro- 
longation of  the  ventricular  systole  after  the  blood  is  expelled. 


10 


Fig  305 —Tracings  of  respiratory  movements  of  individuals  belonging  to  different 
groups  Of  the  animal  kingdom  (after Thanhoffer).  Differences  in  depth,  frequency, 
and  especially  regularity,  are  very  noticeable.  1,  fish;  2,  tortoise;  3,  adder  (nv 
winter);  4,  boa-constrictor  (in  summer);  5,  frog;  0,  alligator;  7,  lizard;  8,  canary- 
bird;  0.  adult  dog;  10,  rabbit;  11,  man;  12,  dog;  13,  horse.  Compare  these,  and 
note  that  in  nl  respiration  is  shallow,  and  in  ml  deep. 


THE   RESPIRATORY   SYSTEM. 


381 


If,  in  the  tracing,  the  small  waves  on  the  upper  part  of  the 
expiratory  curve  really  represent  the  effect  of  the  heart-beat, 
it  makes  it  easier  to  understand  how  such  might  assist  in  venti- 
lating the  blood  when  the  respirations  occur  only  once  in  a 
considerable  interval  and  very  feebly  then,  as  in  hibernating 
animals  or  individuals  that  have  fainted  ;  though  it  must  be 
remembered  that  diffusion  is  a  ceaseless  process  in  all  living 
vertebrates. 


FrG.  306. — Tracings  of  respiration  of  horse  when  at  rest  and  after  exercise  (after  Than- 
hoffer).  /,  inspiration;  E,  expiration.  Spaces  between  vertical  lines  indicate 
time  periods  of  one  second  each.  1,  animal  standing  at  rest;  3,  after  walk  of  few 
minutes;  7  and  8,  after  trotting;  9,  after  a  brief  rest;  11,  after  trotting  and  run- 
ning for  some  minutes;  17,  after  resting  from  last  for  a  short  time;  51,  tracing  at 
end  of  experiment. 

It  is  scarcely  necessary  to  point  out  that  the  respiratory 
movements  are  increased  by  exercise,  emotions,  position,  sea- 
son, hour  of  the  day,  taking  meals,  etc. 

Respiratory  Sounds.— The  entrance  and  exit  of  air  are  ac- 
companied by  certain  sounds,  which  vary  with  eacli  part  of  the 


382  COMPARATIVE   PHYSIOLOGY. 

respiratory  tract.  To  these  sounds  names  have  been  given,  but 
as  they  are  somewhat  inconstant  in  their  application,  or  at  least 
have  several  synonyms,  we  pass  them  by,  recommending  the 
student  to  actually  learn  the  nature  of  the  respiratory  murmurs 
by  listening  to  the  normal  chest  in  both  man  and  the  lower  ani- 
mals. With  the  use  of  a  double  stethoscope  he  may  practice 
upon  himself,  though  not  so  advantageously  as  in  the  case  of 
the  heart. 

The  sounds  are  caused  in  part  by  the  friction  of  the  air, 
though  they  are  probably  complex,  several  factors  entering 
into  their  causation. 


COMPARISON   OF   THE   INSPIRED   AND   EXPIRED  AIR. 

The  changes  that  take  place  in  the  air  respired  may  be  brief- 
ly stated  as  follows : 

1.  Whatever  the  condition  of  the  inspired  air,  that  expired 
is  about  saturated  with  aqueous  vapor — i.  e.,  it  contains  all  that 
it  is  capable  of  holding  at  the  existing  temperature. 

2.  The  temperature  of  the  expired  air  is  about  that  of  the 
blood  itself,  so  that  if  the  air  is  very  cold  when  breathed,  the 
body  loses  a  great  deal  of  its  heat  in  warming  it.  The  expired 
air  of  the  nasal  passages  is  slightly  warmer  than  that  of  the 
mouth. 

3.  Experiment  shows  that  the  expired  air  is  really  dimin- 
ished in  volume  to  the  extent  of  from  one  fortieth  to  one  fif- 
tieth of  the  whole.  Since  two  volumes  of  carbonic  anhydride 
require  for  their  composition  two  volumes  of  oxygen,  if  the 
amount  of  the  former  gas  expired  be  not  equal  to  the  amount 
of  oxygen  inspired,  some  of  the  latter  must  have  been  used  to 

CO 
form  other  combinations.     -=-^,  amounting  to  rather  less  than 

1,  is  called  the  respiratory  coefficient. 

4.  The  difference  between  inspired  and  expired  air  in  man 
may  be  gathered  from  the  following : 


Inspired  air. 
Expired  air. 


Oxygen. 

Nitrogen. 

Carbonic  dioxide. 

20-810 

79-150 

0-040 

10-083 

79-587 

4-380 

From  which  the  most  important  conclusions  to  be  drawn 
are,  that  the  expired  air  is  poorer  in  oxygen  to  the  extent  of  4 
to  5  per  cent,  and  richer  in  carbonic  anhydride  to  somewhat 


THE   RESPIRATORY  SYSTEM.  333 

less  than  this  amount.    A  similar  relationship  may  be  considered 
to  hold  for  the  domestic  animals,  the  quantities  varying,  of  course. 

From  experiment  it  has  been  ascertained  that  the  amount 
of  carbonic  dioxide  is  for  the  average  man  800  grammes  (406 
litres,  equivalent  to  218*1  grammes  carbon)  daily,  the  oxygen 
actually  used  for  the  same  period  being  700  grammes.  But  the 
variations  in  such  cases  are  very  great,  so  that  these  numbers 
must  not  be  interpreted  too  rigidly.  Experience  proves  that, 
while  chemists  often  work  in  laboratories  in  which  the  per- 
centage of  carbonic  anhydride  (from  chemical  decompositions) 
reaches  5  per  cent,  an  ordinary  room  in  which  the  amount  of 
this  gas  reaches  1  per  cent  is  entirely  unfit  for  occupation.  This 
is  not  because  of  the  amount  of  the  carbon  dioxide  present,  but 
of  other  impurities  which  seem  to  be  excreted  in  proportion  to 
the  amount  of  this  gas,  so  that  the  latter  may  be  taken  as  a 
measure  of  these  poisons. 

What  these  are  is  as  yet  almost  entirely  unknown,  but  that 
they  are  poisons  is  beyond  doubt.  Small  effete  particles  of 
once  living  protoplasm  are  carried  out  with  the  breath,  but 
these  other  substances  are  got  rid  of  from  the  blood  by  a  vital 
process  of  secretion  (excretion),  we  must  believe;  which  shows 
that  the  lungs  to  some  degree  play  the  part  of  glands,  and  that 
their  whole  action  is  not  to  be  explained  as  if  they  were  merely 
moistened  bladders  acting  in  accordance  with  ordinary  physi- 
cal laws. 

An  estimation  of  the  amount  of  atmospheric  air  required 
may  be  calculated  from  data  already  given. 

Thus,  assuming  that  a  man  gives  up  at  each  breath  4  per 
cent  of  carbon  dioxide  to  the  500  cc.  of  tidal  air  he  expires,  and 
breathes,  say,  seventeen  times  a  minute,  we  get  for  the  amount 
of  air  thus  charged  in  one  hour  to  the  extent  of  1  per  cent : 
500  x  4  x  17  x  60  =  2,040,000  cc,  or  2,040  litres. 

But  if  the  air  is  to  be  contaminated  to  the  extent  of  only  TV 
per  cent  of  carbonic  anhydride,  the  amount  should  equal  at 
least  2,040  x  10  hourly.  A  very  much  larger  quantity  would, 
of  course,  be  required  for  a  horse  or  an  ox. 


RESPIRATION   IN   THE   BLOOD. 

It  may  be  noticed  that  arterial  blood  kept  in  a  confined  space 
grows  gi'adually  darker  in  color,  and  that  the  original  bright- 
scarlet  hue  may  be  restored  by  shaking  it  up  with  air.     When 


384  COMPARATIVE   PHYSIOLOGY. 

the  blood  has  passed  through  the  capillaries  and  reached  the 
veins,  the  color  has  changed  to  a  sort  of  purple,  characteristic 
of  venous  blood.  Putting  these  two  facts  together,  we  are  led 
to  suspect  that  the  change  has  been  caused  in  some  way  by 
oxygen.  Exact  experiments  with  an  appropriate  form  of  blood- 
pump  show  that  from  one  hundred  volumes  of  blood,  whether 
arterial  or  venous,  about  sixty  volumes  of  gas  may  be  obtained : 
that  this  gas  consists  chiefly  of  oxygen  and  carbonic  anhydride, 
but  that  the  proportions  of  each  present  depends  upon  whether 
the  blood  is  arterial  or  venous. 

The  following  table  will  make  this  clear : 


Arterial  blood 
Venous  blood. 


:ygen.    Cai 

■bonic  anhydride. 

Nitrogen. 

20 

40 

1-2 

8-12 

46 

1-2 

from  100  volumes  of  blood  at  0°  C.  and  760  millimetre  pressure. 

Arterial  blood,  then,  contains  8  to  12  per  cent  more  oxygen 
and  about  6  per  cent  more  carbonic  dioxide  than  venous  blood. 
It  is  not,  of  course,  true,  as  is  sometimes  supposed,  that  arterial 
blood  is  "  pure  blood  "  in  the  sense  that  it  contains  no  carbonic 
anhydride,  as  in  reality  it  always  carries  a  large  percentage  of 
this  gas. 

The  Conditions  under  which  the  Gases  exist  in  the  Blood  — 
If  a  fluid,  as  water,  be  exposed  to  a  mixture  of  gases  which  it 
can  absorb  under  pressure,  it  is  found  that  the  amount  taken  up 
depends  on  the  quantity  of  the  particular  gas  present  independ- 
ent of  the  presence  or  quantity  of  the  others ;  thus,  if  water  be 
exposed  to  a  mixture  of  oxygen  and  nitrogen,  the  quantity  of 
oxygen  absorbed  will  be  the  same  as  if  no  nitrogen  were  pres- 
ent— i.  e. ,  the  absorption  of  a  gas  varies  with  the  partial  press- 
ure of  that  gas  in  the  atmosphere  to  which  it  is  exposed.  But 
whether  blood,  deprived  of  its  gases,  be  thus  exposed  to  oxygen 
under  pressure,  or  whether  the  attempt  be  made  to  remove  this 
gas  from  arterial  blood,  it  is  found  that  the  above-stated  law 
does  not  apply. 

When  blood  is  placed  under  the  exhaustion-pump,  at  first 
very  little  oxygen  is  given  off;  then,  when  the  pressure  is  con- 
siderably reduced,  the  gas  is  suddenly  liberated  in  large  quan- 
tity, and  after  this  comparatively  little.  A  precisely  analogous 
course  of  events  takes  place  when  blood  deprived  of  its  oxygen 
is  submitted  to  this  gas  under  pressure.  On  the  other  hand, 
if  these  experiments  be  made  with  serum,  absorption  follows 


THE   RESPIRATORY  SYSTEM.  385 

according  to  the  law  of  pressures.  Evidently,  then,  if  the  oxy- 
gen is  merely  dissolved  in  the  hlood,  such  solution  is  peculiar, 
and  we  shall  presently  see  that  this  supposition  is  neither  ne- 
cessary nor  reasonable. 


HEMOGLOBIN   AND   ITS   DERIVATIVES. 

Haemoglobin  constitutes  about  -^\  of  the  corpuscles,  and, 
though  amorphous  in  the  living  blood-cells,  may  be  obtained 
in  crystals,  the  form  of  which  varies  with  the  animal ;  indeed, 
in  many  animals  this  substance  crystallizes  spontaneously  on 
the  death  of  the  red  cells.  It  is  unique  among  albuminous 
compounds  in  being  the  only  one  found  in  the  animal  body 
that  is  susceptible  of  crystallization.  Its  estimated  composi- 
tion is: 

Carbon 53'85 

Hydrogen 7"32 

Nitrogen 16  '17 

Oxygen 2184 

Iron -43 

Sulphur -39 

together  with  3  to  4  per  cent  of  water  of  crystallization. 

The  formula  assigned  is:  CeooHgeoOivgN^FeSs.  The  molecu- 
lar constitution  is  not  known,  and  the  above  formula  is  merely 
an  approximation,  which  will,  however,  serve  to  convey  an  idea 
of  the  great  complexity  of  this  compound.  The  presence  of 
iron  seems  to  be  of  great  importance.  If  not  the  essential  re- 
spiratory constituent,  certainly  the  administration  of  this  metal 
in  some  form  proves  very  valuable  when  the  blood  is  deficient 
in  haemoglobin. 

This  substance  can  be  recognized  most  certainly  by  the  spec- 
troscope. The  appearances  vary  with  the  strength  of  the  solu- 
tion, and,  as  this  test  for  blood  (haemoglobin)  is  of  much  prac- 
tical importance,  it  will  be  necessary  to  dwell  a  little  upon  the 
subject;  though,  after  a  student  has  once  recognized  clearly  the 
differences  of  the  spectrum  appearances,  he  has  a  sort  of  knowl- 
edge that  no  verbal  description  can  convey.  This  is  easily  ac- 
quired. One  only  needs  a  small,  flat-sided  bottle  and  a  pocket- 
spectroscope.  Filling  the  bottle  half  full  of  water,  and  getting 
the  spectroscope  so  focused  that  the  Fraunhofer  lines  appear 
distinctly,  blood,  blood-stained  serum,  a  solution  of  haemoglo- 
bin crystals,  or  the  essential  substance  in  any  form  of  dilute 
25 


386 


COMPARATIVE   PHYSIOLOGY. 


solution,  may  be  added  drop  by  drop  till  changes  in  the  spec- 
trum in  the  form  of  dark  bands  appear.  By  gradually  increas- 
ing the  quantity,  ap- 
pearances like  those  fig- 
ured below  may  be  ob- 
served, though,  of  course, 
much  will  depend  on  the 
thickness  of  the  layer  of 
fluid  as  to  the  quantity 
to  be  added  before  a  par- 
ticular band  comes  into 
view. 

When  wishing  to  be 
precise,  we  speak  of  the 
most  highly  oxidized 
form  of  haemoglobin  as 
oxy-haemoglobin  (O-H), 
and  the  reduced  form  as 
haemoglobin  simply,  or 
reduced  haemoglobin  (H) . 
By  a  comparison  of 
the  spectra  it  will  be  seen 
that  the  bands  of  oxy- 
haemoglobin  lie  between 
the  D  and  E lines;  that 
the  left  band  near  D  is 
always  the  most  definite 
in  outline  and  the  most 
pronounced  in  every  re- 
spect except  breadth ;  that  it  is  in  weak  solutions  the  first  to  ap- 
pear, and  the  last  to  disappear  on  reduction ;  that  there  are  two 
instances  in  which  there  may  be  a  single  band  from  haemoglo- 
bin— in  the  one  case  when  the  solution  is  very  dilute  and  when 
it  is  very  concentrated.  These  need  never  be  mistaken  for  each 
other  nor  for  the  band  of  reduced  haemoglobin.  The  latter  is  a 
hazy  broad  band  with  comparatively  indistinct  outlines,  and 
darkest  in  the  middle. 

It  will  be  further  noticed  that  in  all  these  instances,  apart 
from  the  bands,  the  spectrum  is  otherwise  modified  at  each 
end,  so  that  the  darker  the  more  centrally  placed  characteristic 
bands,  the  more  is  the  light  at  the  same  time  cut  off  at  each 
end  of  the  spectrum. 


Fig.  307.— Crystallized  haemoglobin  (Gautier).  a,  b, 
crystals  from  venous  blood  of  man;  c,  from 
blood  of  cat;  d,  of  Guinea-pig;  e,  of  marmot; 
f,  of  squirrel. 


THE  RESPIRATORY   SYSTEM. 


387 


388  COMPARATIVE  PHYSIOLOGY. 

If,  now,  to  a  specimen  showing  the  two  bands  of  oxy-haemo- 
globin  distinctly  a  few  drops  of  ammonium  sulphide  or  other 
reducing  agent  be  added,  a  change  in  the  color  of  the  solution 
will  result,  and  the  single  hazy  band  characteristic  of  hemo- 
globin will  appear. 

It  is  not  to  be  supposed,  however,  that  venous  blood  gives 
this  spectrum.  Even  after  asphyxia  it  will  be  difficult  to  see 
this  band,  for  usually  some  of  the  oxy-ha^moglobin  remains 
reduced ;  but  it  is  worthy  of  note,  as  showing  that  the  appear- 
ances are  normal,  that  the  blood,  viewed  through  thin  tissues 
when  actually  circulating,  whether  arterial  or  venous,  gives 
the  spectrum  of  oxy-bsemoglobin.  At  the  same  time  there  can 
be  no  doubt  that  the  changes  in  color  which  the  blood  under- 
goes in  passing  through  the  capillaries  is  due  chiefly  to  loss  of 
oxygen,  as  evidenced  by  the  experiments  before  referred  to ;  and 
the  reason  that  tbe  two  bands  are  always  to  be  seen  in  venous 
blood  is  simply  that  enough  oxy-haamoglobin  remains  to  give 
the  two-band  spectrum  which  prevails  over  that  of  (reduced) 
haemoglobin.  We  are  thus  led  by  many  paths  to  the  important 
conclusion  that  the  red  corpuscles  are  oxygen-carriers,  and, 
though  this  may  not  be  and  probably  is  not  their  only  func- 
tion, it  is  without  doubt  their  principal  one.  Of  their  oxygen 
they  are  being  constantly  relieved  by  the  tissues;  hence  the 
necessity  of  a  circulation  of  the  blood  from  a  respiratory  point 
of  view. 

There  are  other  gases  that  can  replace  oxygen  and  form 
compounds  with  haemoglobin ;  hence  we  have  CO-hasmoglobin 
and  NO-haemoglobin,  which  in  turn  are  replaced  by  oxygen  with 
no  little  difficulty — a  fact  which  explains  why  carbonic  oxide  is 
so  fatal  when  respired,  and,  as  it  is  a  constituent  of  illuminat- 
ing gas,  the  cause  of  the  death  of  those  inhaling  the  latter  is 
often  not  far  to  seek.  Blood  may,  in  fact,  be  saturated  with 
carbonic  oxide  by  allowing  illuminating  gas  to  pass  through  it, 
when  a  change  of  color  to  a  cherry  red  may  be  observed,  and 
which  will  remain  in  spite  of  prolonged  shaking  up  with  air  or 
attempts  at  reduction  with  the  usual  reagents.  Haemoglobin 
may  be  resolved  into  a  proteid  (globin)  not  well  understood, 
and  hcematin.  This  happens  when  the  blood  is  boiled  (perhaps 
also  in  certain  cases  of  lightning-stroke),  and  when  strong  acids 
are  added.  Haematin  is  soluble  in  dilute  acids  and  alkalies,  and 
has  then  characteristic  spectra.  Alkaline  haematin  may  be  re- 
duced ;  and,  as  the  iron  can  be  separated,  resulting  in  a  change 


THE  RESPIRATORY   SYSTEM.  389 

of  color  to  brownish  red,  after  which  there  are  no  longer  any 
reducing-  effects,  it  would  seem  that  the  oxygen-carrying  power 
and  iron  are  associated.  This  iron-free  haernatin  is  named 
hcematorporphyrin  or  hcematoin. 

Hcemin  is  hydrochlorate  of  haernatin  (Teichinanms  crystals), 
and  may  be  formed  by  adding  glacial  acetic  acid  and  common 
salt  to  blood,  dried  blood-clot,  etc.,  and  heating  to  boiling.  This 
is  one  of  the  best  tests  for  blood,  valuable  in  medico-legal  and 
other  cases. 

When  oxy-haemoglobin  stands  exposed  to  the  air,  or  when 
diffused  in  urine,  it  changes  color  and  becomes,  in  fact,  another 
substance — methcemoglobin,  irreducible  by  other  gases  (CO,  etc.), 
and  not  surrendering  its  oxygen  in  vacuo,  though  giving  it  up 
to  ammonium  sulphide,  becoming  again  oxy-haemoglobin,  when 
shaken  up  with  atmospheric  air.  Its  spectrum  differs  from 
that  of  oxy-haemoglobin  in  that  it  has  a  band  in  the  red  end  of 
the  spectrum  between  the  C  and  D  lines.  Hcematoidin  is  some- 
times found  in  the  body  as  a  remnant  of  old  blood-clots.  It  is 
probably  closely  allied  to  if  not  identical  with  the  bilirubin 
of  bile. 

Comparative. — "While  haemoglobin  is  the  respiratory  agent  in 
all  the  groups  of  vertebrates,  this  is  not  true  of  the  inverte- 
brates. Red  blood-cells  have  as  yet  been  found  in  but  a  few 
species,  though  haemoglobin  does  exist  in  the  blood  plasma  of 
several  groups,  to  one  of  which  the  earth-worm  and  several 
other  annelids  belong.  It  is  interesting  to  note  that  the  respir- 
atory compound  in  certain  families  of  crustaceans,  as  the  com- 
mon crab,  horseshoe-crab  (limulus),  etc.,  is  blue,  and  that  in 
this  substance  copper  seems  to  take  the  place  of  iron. 

The  Nitrogen,  and  the  Carbon  Dioxide  of  the  Blood. -The 
little  nitrogen  which  is  found  in  about  equal  quantity  in  venous 
and  arterial  blood  seems  to  be  simply  dissolved.  The  relations 
of  carbonic  anhydride  are  much  more  complex  arid  obscure. 
The  main  facts  known  are  that — 1.  The  quantity  of  tin's  gas  is 
as  great  in  serum  as  in  blood,  or,  at  all  events,  the  quantity  in 
serum  is  very  large.  2.  The  greater  part  may  be  extracted  by 
an  exhaustion-pump ;  but  a  small  percentage  (2  to  5  volumes 
per  cent)  does  not  yield  to  this  method,  but  is  given  off  when 
an  acid  is  added  to  the  serum.  3.  If  the  entire  blood  be  sub- 
jected to  a  vacuum,  the  whole  of  the  C02  is  given  off. 

From  these  facts  it  has  been  concluded  that  the  greater  part 
of  the  C02,  exists  in  the  plasma,  associated  probably  with  sodium 


390  COMPARATIVE   PHYSIOLOGY. 

salts,  as  sodium  bicarbonate,  but  that  the  corpuscles  in  some  way 
determine  its  relations  of  association  and  disassociation.  Some 
think  a  good  deal  of  this  gas  is  actually  united  with  the  red  cor- 
puscles. 

We  may  now  inquire  into  the  more  intimate  nature  of  respi- 
ration in  the  blood.  From  the  facts  we  have  stated  it  is  obvious 
that  respiration  can  not  be  wholly  explained  by  the  Henry-Dal- 
ton  law  of  pressures  or  any  other  physical  law.  It  is  also  plain 
that  any  explanation  which  leaves  out  the  principle  of  pressure 
must  be  incomplete. 

While  there  is  in  oxy-haemoglobin  a  certain  quantity  of 
oxygen,  which  is  intra-molecular  and  incapable  of  removal  by 
reduction  of  pressure,  there  is  also  a  portion  which  is  subject 
to  this  law,  though  in  a  peculiar  way ;  nor  is  tbe  question  of 
temperature  to  be  excluded,  for  experiment  shows  that  less 
oxygen  is  taken  up  by  blood  at  a  high  than  at  a  low  tempera- 
ture. 

We  have  learned  that  in  ordinary  respiration,  the  propor- 
tion of  carbonic  dioxide  and  oxygen  in  different  parts  of  the 
respiratory  tract  must  vary  greatly ;  the  air  of  necessity  being 
much  less  pure  in  the  alveoli  than  in  the  larger  bronchi. 

It  is  customary  to  speak  of  the  oxygen  of  oxy-hsemoglobin 
as  being  in  a  state  of  u  loose  chemical  combination."  The  en- 
tire truth  seems  to  lie  in  neither  view,  though  both  are  partially 
correct.  The  view  entertained  by  some  physiologists,  to  the 
effect  that  diffusion  explains  the  whole  matter,  so  far,  at  least, 
as  carbonic  anhydride  is  concerned,  and  that  the  epithelial  cells 
of  the  lung  have  no  share  in  the  respiratory  process,  does  not 
seem  to  be  in  harmony  either  with  the  facts  of  respiration  or 
with  the  laws  of  biology  in  general.  Why  not  say  at  once  that 
the  facts  of  respiration  show  that,  here  as  in  other  parts  of  the 
economy,  while  physical  and  chemical  laws,  as  we  know  them, 
stand  related  to  the  vital  processes,  yet,  by  reason  of  being  vital 
processes,  we  can  not  explain  them  according  to  the  theories  of 
either  physics  or  chemistry  ?  Surely  this  very  subject  shows 
that  neither  chemistry  nor  physics  is  at  present  adequate  to 
explain  such  processes.  It  is,  of  course,  of  value  to  know  the 
circumstances  of  tension,  temperature,  etc.,  under  which  respi- 
ration takes  place.  We,  however,  maintain  that  these  are  con- 
ditions only — essential  no  doubt,  but  though  important,  that 
they  do  not  make  up  the  process  of  respiration.  But,  because 
we  do  not  know  the  real  explanation,  let  us  not  exalt  a  few 


THE   RESPIRATORY   SYSTEM.   '  39 1 

facts  or  theories  of  chemistry  or  physics  into  a  solution  of  a 
complex  problem.  Besides,  some  of  the  experiments  on  which 
the  conclusions  have  been  based  are  questionable,  inasmuch  as 
they  seem  to  induce  artificial  conditions  in  the  animals  oper- 
ated upon ;  and  we  have  already  insisted  on  the  blood  being 
regarded  as  a  living  tissue,  behaving  differently  in  the  body 
and  when  isolated  from  it,  so  that  even  in  so-called  blood-gas 
experiments  there  may  be  sources  of  fallacy  inherent  in  the 
nature  of  the  case. 

Foreign  Gases  and  Respiration.— These  are  divided  into: 

1.  Indifferent  gases,  as  N,  H,  CH4,  which  though  not  in 
themselves  injurious,  are  entirely  useless  to  the  economy. 

2.  Poisonous  gases,  fatal,  no  matter  how  abundant  the  nor- 
mal respiratory  food  may  be.  They  are  divisible  into :  (a)  those 
that  kill  by  displacing  oxygen,  as  NO,  CO,  HCN ;  (b)  narcotic 
gases,  as  C02,  N20,  producing  asphyxia  when  present  in  large 
quantities;  (c)  reducing  gases,  as  H2S,  (NH4)2S,  PH3,  AsH3,  C2N2, 
which  rob  the  haemoglobin  of  its  oxygen. 

There  are  probably  a  number  of  poisonous  products,  some 
of  them  possibly  gases,  produced  by  the  tissues  themselves  and 
eliminated  normally  by  the  respiratory  tract  ;  and  these  are 
doubtless  greatly  augmented,  either  in  number  or  quantity,  or 
both,  when  other  excreting  organs  are  disordered. 

RESPIRATION   IN   THE    TISSUES. 

We  first  direct  attention  to  certain  striking  facts : 
1.  An  isolated  (frog's)  muscle  will  continue  to  contract  for 
a  considerable  period  and  to  exhale  carbon  dioxide  in  the  total 
absence  of  oxygen,  as  in  an  atmosphere  of  hydrogen ;  though, 
of  course,  there  is  a  limit  to  this,  and  a  muscle  to  which  either 
no  blood  flows,  or  only  venous  blood,  soon  shows  signs  of 
fatigue.  2.  In  a  frog,  in  which  physiological  saline  solution 
has  been  substituted  for  blood,  the  metabolism  will  continue, 
carbonic  anhydride  being  exhaled  as  usual.  3.  Substances, 
which  are  readily  oxidized,  when  introduced  into  the  blood  of 
a  living  animal  or  into  that  blood  when  withdrawn  uudergo 
but  little  oxidative  change.  4.  An  entire  frog  will  respire  car- 
bonic dioxide  for  hours  in  an  atmosphere  of  nitrogen. 

Such  facts  as  these  seem  to  teach  certain  lessons  clearly.  It 
is  evident,  first  of  all,  that  the  oxidative  processes  that  give  rise 
to  carbon  dioxide  occur  chiefly  in  the  tissues  and  not  in  the 


392  COMPARATIVE  PHYSIOLOGY. 

blood ;  that  in  the  case  of  muscle  the  oxygen  that  is  used  is  first 
laid  by,  banked  as  it  were  against  a  time  of  need,  in  the  form  of 
infra-molecular  oxygen,  which  is  again  set  free  in  the  form  of 
carbon  dioxide,  but  by  what  series  of  changes  we  are  quite  un- 
able to  say.  Though  our  knowledge  of  the  respiratory  processes 
of  muscle  is  greater  than  for  any  other  tissue,  there  seems  to 
be  no  reason  to  believe  that  they  are  essentially  different  else- 
where. The  advantages  of  this  banking  of  oxygen  are,  of 
course,  obvious ;  were  it  otherwise,  the  life  of  every  cell  must 
be  at  the  mercy  of  the  slightest  interruption  of  the  flow  of 
blood,  the  entrance  of  air,  etc.  Even  as  it  is,  the  need  of  a 
constant  supply  of  oxygen  in  warm-blooded  animals  is  much 
greater  than  in  cold-blooded  creatures,  which  can  long  endure 
almost  entire  cessation  of  both  respiration  and  circulation, 
owing  to  the  comparatively  slow  rate  of  speed  of  the  vital 
machinery. 

If  one  were  to  rely  on  mere  appearances  he  might  suppose 
that  in  the  more  active  condition  of  certain  organs  there  was 
less  chemical  interchange  (respiration)  between  the  blood  aud 
the  tissues  than  in  the  resting  stage,  or,  properly  speaking, 
more  tranquil  stage,  for  it  must  be  borne  in  mind  that  a  living 
cell  is  never  wholly  at  rest  ;  its  molecular  changes  are  cease- 
less. It  happens,  e.  g.,  that  when  certain  glands  (salivary)  are 
secreting  actively,  the  blood  flowing  from  them  is  less  venous 
in  appearance  than  when  not  functionally  active.  This  is  not 
because  less  oxygen  is  used  or  less  abstracted  from  the  blood, 
but  because  of  the  greatly  increased  speed  of  the  blood-flow,  so 
that  the  total  supply  to  draw  from  is  so  much  larger  that, 
though  more  oxygen  is  actually  used,  it  is  not  so  much  missed, 
nor  do  the  greater  additions  of  carbon  dioxide  so  rapidly  pol- 
lute this  rapid  stream. 

It  is  thus  seen  that  throughout  the  animal  kingdom  respira- 
tion is  fundamentally  the  same  process.  It  is  in  every  case 
finally  a  consumption  of  oxygen  and  production  of  carbonic 
anhydride  by  the  individual  cell,  whether  that  be  an  Amoeba 
or  an  element  of  man's  brain.  These  are,  however,  but  the 
beginning  and  end  of  a  very  complicated  biological  history  of 
by  far  the  greater  part  of  which  nothing  is  yet  known  ;  and  it 
must  be  admitted  that  diffusion  or  any  physical  explanation 
carries  us  but  a  little  way  on  toward  the  understanding  of  it. 


THE  RESPIRATORY  SYSTEM.  393 

THE   NERVOUS    SYSTEM  IN   RELATION   TO   RESPIRA- 
TION. 

We  have  considered  the  muscular  movements  by  which  the 
air  is  made  to  enter  and  leave  the  lungs  in  consequence  of 
changes  in  the  diameters  of  the  air-inclosing  case,  the  thorax. 
It  remains  to  examine  into  the  means  by  which  these  muscles 
were  set  into  harmonious  action  so  as  to  accomplish  the  pur- 
pose. The  nerves  supplying  the  muscles  of  respiration  are  de- 
rived from  the  spinal  cord,  so  that  they  must  be  under  the  do- 
minion of  central  nerve-cells  situated  either  in  the  cord  or  the 
brain.  Is  the  influence  that  proceeds  outward  generated  within 
the  cells  independently  of  any  afferent  impulses,  or  is  it  depend- 
ent on  such  causes  ? 

A  host  of  facts,  experimental  and  other,  show  that  the  cen- 
tral impulses  are  modified  by  afferent  impulses  reaching  the 
center  through  appropriate  nerves.  Moreover,  drugs  seem  to 
act  directly  on  the  center  through  the  blood. 

The  vagus  is  without  doubt  the  afferent  respiratory  nerve, 
though  how  it  is  affected,  whether  by  the  mechanical  movement 
of  the  lungs,  merely,  by  the  condition  of  the  blood  as  regards 
its  contained  gases,  or,  as  seems  most  likely,  by  a  combination 
of  circumstances  into  which  these  enter  and  are  probably  the 
principal,  is  not  demonstrably  clear.  When  others  function  as 
afferent  nerves,  capable  of  modifying  the  action  of  the  respira- 
tory center,  they  are  probably  influenced  by  the  respiratory 
condition  of  the  blood,  though  not  necessarily  exclusively. 

But  when  all  the  principal  afferent  impulses  are  cut  off  by 
division  of  the  nerves  reaching  the  respiratory  center  directly 
or  indirectly,  respiration  will  still  continue,  provided  the  motor 
nerves  and  the  medulla  remain  intact. 

The  center,  then,  is  not  mainly  at  least,  a  reflex  but  an  auto- 
matic one,  though  its  action  is  modified  by  afferent  impulses 
reaching  it  from  every  quarter. 

It  has  been  argued  that  there  are  both  inspiratory  and  ex- 
piratory centers  in  the  spinal  cord,  but  this  can  not  yet  be  re- 
garded as  established.  But,  as  we  have  pointed  out,  on  more 
than  one  occasion,  we  must  always  be  on  our  guard  in  inter- 
preting the  behavior  of  one  part  when  another  is  out  of  gear. 

The  Influence  of  the  Condition  of  the  Blood  in  Respiration. — 
If  for  any  reason  the  tissues  are  not  receiving  a  due  supply  of 
oxygen,  they  manifest  their  disapproval,  to  speak  figuratively, 


394 


COMPARATIVE   PHYSIOLOGY. 


Brain  above  medulla  from  which 
impulses  modifying  respiration 
may  proceed. 


'acial  muscles. 


Respiratory  centre 
in  the  medulla. 


Cutaneous  surface  from  which 
afferent  impulses  proceed  cZn 
rectly  to  brain. 


---I — Thoracic  resp.  muscles. 


Spinal  cord.— 


r— Respiratory  tract. 


Diaphragm  with 
phrenic  nerve. 


Cutaneous  sur- 
face from  which 
impulses  reach  res- 
piratory centre  by 
spinal  cord. 


Fig.  809.— Diagram  intended  to  illustrate  nervous  mechanism  of  respiration.    Arrows 
indicate  course  of  impulses. 


THE  RESPIRATORY   SYSTEM.  395 

by  reports  to  the  responsible  center  in  the  medulla,  and  if  the 
medulla  is  a  sharer  in  the  lack,  as  it  naturally  would  be,  it  takes 
action  independently.  One  of  the  most  obvious  instances  in 
which  there  is  oxygen  starvation  is  when  there  is  hindrance  to 
the  entrance  of  air,  owing  to  obstruction  in  the  respiratory  tract. 

At  first  the  breathing  is  merely  accelerated,  with  perhaps 
some  increase  in  the  depth  of  the  inspirations  (hyperpnoea),  a 
stage  which  is  soon  succeeded  by  labored  breathing  (dyspnoea), 
which,  after  the  medulla  has  called  all  the  muscles  usually  em- 
ployed in  respiration  into  violent  action,  passes  into  convul- 
sions, in  which  every  muscle  may  take  part. 

In  other  words,  the  respiratory  impulses  not  only  pass  along 
their  usual  paths  as  energetically  as  possible,  but  radiate  into 
unusual  ones  and  pass  by  nerves  not  commonly  thus  set  into 
functional  activity. 

It  would  be  more  correct,  perhaps,  to  assume  that  the  vari- 
ous parts  of  the  nervous  system  are  so  linked  together  that  ex- 
cessive activity  of  one  set  of  connections  acts  like  a  stimulus  to 
rouse  another  set  into  action,  the  order  in  which  this  happens 
depending  on  the  law  of  habit — habit  personal  and  especially 
ancestral.  An  opposite  condition  to  that  described,  known  as 
apnoea,  may  be  induced  by  pumping  air  into  an  amimal's  chest 
very  rapidly  by  a  bellows ;  or  in  one's  self  by  a  succession  of 
rapid,  deep  respirations. 

After  ceasing,  the  breathing  may  be  entirely  interrupted 
for  a  brief  interval,  then  commence  very  quietly,  gradually  in- 
creasing to  the  normal. 

Apnoea  has  been  interpreted  in  two  ways.  Some  think  that 
it  is  due  to  fatigue  of  the  muscles  of  respiration  or  the  respira- 
tory center;  others  that  the  blood  has  under  these  circum- 
stances an  excess  of  oxygen,  which  so  influences  the  respiratory 
center  that  it  is  quieted  (inhibited)  for  a  time. 

The  latter  view  is  that  usualty  adopted ;  but  considering  that 
apnoea  results  from  the  sobbing  of  children  following  a  pro- 
longed fit  of  crying,  also  in  Cheyne-Stokes  and  other  abnormal 
forms  of  breathing,  and  that  the  blood  is  normally  almost  satu- 
rated with  oxygen,  it  will  be  agreed  that  there  is  a  good  deal  to 
be  said  for  the  first  view,  especially  that  part  of  it  which  repre- 
sents the  cessation  of  breathing  as  owing  to  excessive  activity 
and  exhaustion  of  the  respiratory  center.  We  find  such  a  calm 
in  asphyxia  after  the  convulsive  storm.  Perhaps  if  we  regard 
the  respiratory  center  as  double,  half  being  situated  on  each  side 


396  COMPARATIVE  PHYSIOLOGY. 

of  the  middle  Hue ;  also  as  made  up  of  au  inspiratory  and  ex- 
piratory part;  automatic  essentially,  but  greatly  modified  by 
afferent  impulses,  especially  those  ascending  the  vagi  nerves; 
while  the  latter  may  be  considered  as  containing  both  inhib- 
itory and  augmenting  fibers  for  the  center,  the  whole  process 
will  be  clearer.  Respiration  on  this  view  would  be  self -regula- 
tive ;  the  deeper  the  inspiration,  the  stronger  the  inhibitory  in- 
fluence, so  the  greater  the  tendency  to  arrest  of  inspiration; 
hence  either  expiration  or  apncea. 

Is  it,  then,  the  excessive  accumulation  of  carbon  dioxide  or 
the  deficiency  of  oxygen  that  induces  dyspnoea  ?  Considering 
that  the  former  gas  acts  as  a  narcotic,  and  does  not  induce  con- 
vulsions, even  when  it  constitutes  a  large  percentage  of  the 
atmosphere  breathed,  and  that  the  need  of  oxygen  for  the  tis- 
sues is  constant,  it  certainly  seems  most  reasonable  to  conclude 
that  the  phenomena  of  dyspnoea  are  owing  to  the  lack  of  oxy- 
gen, chiefly  at  least;  though  the  presence  of  an  excess  of  car- 
bonic anhydride  may  take  some  share  in  arousing  that  vigorous 
effort  on  the  part  of  the  nervous  system,  to  restore  the  func- 
tional equilibrium,  so  evident  under  the  circumstances. 

THE    INFLUENCE    OF    RESPIRATION    ON    THE 
CIRCULATION. 

An  examination  of  tracings  of  the  intra-thoracic  and  blood- 
pressure,  taken  simultaneously,  shows  (1)  that  during  inspira- 
tion the  blood-pressure  rises  and  the  intra-thoracic  pressure 
falls  ;  (2)  that  during  expiration  the  reverse  is  true  ;  and  (3) 
that  the  heart-beat  is  slowed,  and  has  a  decided  effect  on  the 
form  of  the  pulse.  But  it  also  appears  that  the  period  of  high- 
est blood-pressure  is  just  after  expiration  has  begun. 

We  must  now  attempt  to  explain  how  these  changes  are 
brought  about.  By  intra-thoracic  pressure  is  meant  the  press- 
ure the  lungs  exert  on  the  costal  pleura  or  any  organ  within 
the  chest,  which  must  differ  from  intra-pulmonary  pressure 
and  the  pressure  of  the  atmosphere,  because  of  the  resistance  of 
the  lungs  by  virtue  of  their  own  elasticity. 

It  has  been  noted  that  even  in  death  the  lungs  remain  par- 
tially distended  ;  and  that  when  the  thorax  is  opened  the  pul- 
monary collapse  wbich  follows  demonstrates  that  their  elas- 
ticity amounts  to  about  five  millimetres  of  mercury,  which 
must,  of  course,  represent  but  a  small  portion  of  that  elasticity 


THE   RESPIRATORY   SYSTEM. 


397 


which  may  he  brought  into  play  when  these  organs  are  greatly 
distended,  so  that  they  never  press  on  the  costal  walls,  heart, 


Fig.  310. — Tracings  of  blood-pressure  and  intrathoracic  pressure  in  the  dog  (after 
Foster),  a,  blood-pressure  tracing  showing  irregularities  due  to  respiration  and 
pulse;  b,  curve  of  intrathoracic  pressure;  i,  beginning  of  inspiration;  e,  of  expira- 
tion. Intrathoracic  pressure  is  seen  to  rise  rapidly  after  inspiration  ceases,  and 
then  slowly  sinks  as  the  expiratory  blast  continues,  to  become  a  rapid  fall  when 
inspiration  begins. 

etc.,  with  a  pressure  equal  to  that  of  the  atmosphere.  It  follows 
that  the  deeper  the  inspiration  the  greater  the  difference  be- 
tween the  intra-thoracic  and  the  atmospheric  pressure.  Even 
in  expiration,  except  when  forced,  the  intra-thoracic  pressure 
remains  less,  for  the  same  reason. 

These  conditions  must  have  an  influence  on  the  heart  and 
blood-vessels.  Bearing  in  mind  that  the  pressure  without  is 
practically  constant  and  always  greater  than  that  within  the 
thorax,  the  conditions  are  favorable  to  the  flow  of  blood  toward 
the  heart.  As  in  inspiration,  the  pressure  on  the  great  veins 
and  the  heart  is  diminished,  and,  as  these  organs  are  not  rigid, 
they  tend  to  expand  within  the  thorax,  thus  favoring  an  on- 
ward flow.  But  the  opposite  effect  would  follow  as  regards  the 
large  arteries.  Their  expansion  must  tend  to  withdraw  blood. 
During  expiration  the  conditions  are  reversed.  The  effects  on 
the  great  veins  can  be  observed  by  laying  them  bare  in  the 
neck  of  an  animal,  when  it  may  be  seen  that  during  inspiration 
they  become  partially  collapsed,  and  refilled  during  expiration. 
In  consequence  of  the  marked  thickness  of  the  coats  of  the 
great  arteries,  the  effect  of  changes  in  intra-thoracic  pressure 
must  be  slight.  The  comparatively  thin-walled  auricles  act 
somewhat  as  the  veins,  and  it  is  likely  that  the  increase  of 
pressure  during  expiration  must  favor,  so  far  as  it  goes,  the  car- 
diac systole. 


398  COMPARATIVE   PHYSIOLOGY. 

More  blood,  then,  entering  the  right  side  of  the  heart  dur- 
ing inspiration,  more  will  be  thrown  into  tbe  systemic  circula- 
tion, unless  it  be  retained  in  the  lungs,  and,  unless  the  effect  be 
counteracted,  the  arterial  pressure  will  rise,  and,  as  all  the  con- 
ditions are  reversed  during  expiration,  we  look  for  and  find 
exactly  opposite  results.  The  lungs  themselves,  however,  must 
be  taken  into  the  account.  During  inspiration  room  is  pro- 
vided for  an  increased  quantity  of  blood,  the  resistance  to  its 
flow  is  lessened,  hence  more  blood  reaches  the  left  side  of  the 
heart.  The  immediate  effect  would  be,  notwithstanding,  some 
diminution  in  the  quantity  flowing  to  the  left  heart,  in  conse- 
quence of  the  sudden  widening  of  the  pulmonary  vessels,  the 
reverse  of  which  would  follow  during  expiration  ;  hence  the 
period  of  highest  intra-thoracic  pressure  is  after  the  onset  of 
the  expiratory  act.  During  inspiration  the  descent  of  the 
diaphragm  compressing  the  abdominal  organs  is  thought  to 
force  on  blood  from  the  abdominal  veins  into  the  thoracic  vena 
cava. 

That  the  respiratory  movements  do  exert  in  some  way  a 
pronounced  effect  on  the  circulation  the  student  may  demon- 
strate to  himself  in  the  following  ways  :  1.  After  a  full  inspira- 
tion, close  the  glottis  and  attempt  to  expire  forcibly,  keeping 
the  fingers  on  the  radial  artery.  It  may  be  noticed  that  the 
pulse  is  modified  or  possibly  for  a  moment  disappears.  2.  Re- 
verse the  experiment  by  trying  to  inspire  forcibly  with  closed 
glottis  after  a  strong  expiration,  when  the  pulse  will  again  be 
found  to  vary.  In  the  first  instance,  the  heart  is  comparatively 
empty  and  hampered  in  its  action,  intra-thoracic  pressure  being 
so  great  as  to  prevent  the  entrance  of  venous  blood  by  com- 
pression of  the  heart  and  veins,  while  that  already  within  the 
organ  and  returning  to  it  from  the  lungs  soon  passes  on  into 
the  general  system,  hence  the  pulseless  condition.  The  expla- 
nation is  to  be  reversed  for  the  second  case.  The  heart's  beat  is 
modified,  probably  reflexly,  through  the  cardio-inhibitory  cen- 
ter, for  the  changes  in  the  pulse-rate  do  not  occur  when  the  vagi 
nerves  are  cut,  at  least  not  to  nearly  the  same  extent. 

Comparative.— It  may  be  stated  that  the  cardiac  phenomena 
referred  to  in  this  section  are  much  more  marked  in  some  ani- 
mals than  in  others.  Very  little  change  may  be  observed  in 
the  pulse-rate  in  man,  while  in  the  dog  it  is  so  decided  that  one 
observing  it  for  the  first  time  might  suppose  that  such  pro- 
nounced irregularity  of  the  heart  was  the  result  of  disease  ; 


THE   RESPIRATORY  SYSTEM.  399 

though  even  in  this  animal  there  are  variations  in  this  respect 
with  the  breed,  age,  etc. 

The  Respiration  and  Circulation  in  Asphyxia.— A  most  in- 
structive experiment  may  be  arranged  thus : 

Let  an  anaesthetized  rabbit,  cat,  or  such-like  animal,  have 
the  carotid  of  one  side  connected  with  a  glass  tube  as  before 
desci'ibed  (pages  228,  229),  by  which  the  blood-pressure  and  its 
changes  may  be  indicated,  and,  when  the  normal  respiratory 
acts  have  been  carefully  observed,  proceed  to  notice  the  effects 
on  the  blood-pressure,  etc.,  of  pumping  air  into  the  chest  by  a 
bellows,  of  hindering  the  ingress  of  air  to  a  moderate  degree, 
and  of  struggling.  "With  a  small  animal  it  will  be  difficult  to 
observe  the  respiratory  effects  on  the  blood-pressure  by  simply 
watching  the  oscillations  of  the  fluid  in  the  glass  tube,  but  this 
is  readily  enough  made  out  if  more  elaborate  arrangements  be 
made,  so  that  a  graphic  tracing  may  be  obtained. 

But  the  main  events  of  asphyxia  may  be  well  (perhaps  best) 
studied  in  this  manner: 

Let  the  trachea  be  occluded  (ligatured).  At  once  the  blood- 
pressure  will  be  seen  to  rise  and  remain  elevated  for  some  time, 
then  gradually  fall  to  zero.  These  changes  are  contemporane- 
ous with  a  series  of  remarkable  manifestations  of  disturbance 
in  the  respiratory  system  as  it  at  first  appears,  but  in  reality 
due  to  wide-spread  and  profound  nutritive  disturbance.  So  far 
as  the  breathing  is  concerned,  it  may  be  seen  to  become  more 
rapid,  deeper,  and  labored,  in  which  the  expiratory  phase  be- 
comes more  than  proportionably  marked  (dyspnoea) ;  this  is  fol- 
lowed by  the  gradual  action  of  other  muscles  than  those  usually 
employed  in  respiration,  until  the  whole  body  passes  into  a  ter- 
rible convulsion — a  muscle-storm  in  consequence  of  a  nerve- 
storm.  When  this  has  lasted  a  variable  time,  but  usually 
about  one  minute,  there  follows  a  period  of  exhaustion,  during 
which  the  subject  of  the  experiment  is  in  a  motionless  condi- 
tion, interrupted  by  an  occasional  respiration,  in  which  inspi- 
ration is  more  pronounced  than  expiration;  and,  finally,  the 
animal  quietly  stretches  every  limb,  the  sphincters  are  relaxed, 
there  may  be  a  discharge  of  urine  or  faeces  from  peristaltic 
movements  of  the  bladder  or  intestines,  and  death  ends  a  strik- 
ing scene.  These  events  may  be  classified  in  three  stages, 
though  the  first  and  second  especially  merge  into  one  another: 
1.  Stage  of  dyspnoea.  2.  Stage  of  convulsions.  3.  Stage  of 
exhaustion. 


400  COMPARATIVE   PHYSIOLOGY.  • 

It  is  during  the  first  two  stages  that  the  blood-pressui'e  rises, 
and  during  the  third  that  it  sinks,  due  in  the  first  instance 
chiefly  to  excessive  activity  of  the  vaso-motor  center,  and  in 
the  second  to  its  exhaustion  and  the  weakening  of  the  heart- 
beat. 

These  violent  movements  are  owing,  we  repeat,  to  the  action 
of  blood  deficient  in  oxygen  on  the  respiratory  center  (or  cen- 
ters), leading  to  inordinate  action  followed  by  exhaustion. 

The  duration  of  the  stages  of  asphyxia  varies  with  the  ani- 
mal, but  rarely  exceeds  five  minutes.  In  this  connection  it  may 
be  noted  that  newly  born  animals  (kittens,  puppies)  bear  im- 
mersion in  water  for  as  much  as  from  thirty  to  fifty  minutes, 
while  an  adult  dog  dies  within  four  or  five  minutes.  This  is 
to  be  explained  by  the  feeble  metabolism  of  new-born  mam- 
mals, which  so  slowly  uses  up  the  vital  air  (oxygen). 

If  the  chest  of  an  animal  be  opened,  though  the  respiratory 
muscles  contract  as  usual  there  is,  of  course,  no  ventilation  of 
the  lungs  which  lie  collapsed  in  the  chest ;  and  the  animal  dies 
about  as  quickly  as  if  its  trachea  were  occluded.  It  passes 
through  all  the  phases  of  asphyxia  as  in  the  former  case ;  but 
additional  information  may  be  gained.  The  heart  is  seen  to 
beat  at  first  more  quickly  and  forcibly,  later  vigorously  though 
slower,  and  finally  both  feebly  and  irregularly,  till  the  ventri- 
cles, then  the  left  auricle,  and  finally  tho  right  auricle  cease  to 
beat  at  all  or  only  at  long  intervals.  The  terminations  of  the 
great  veins  (representing  the  sinus  venosus)  beat  last  of  all. 

At  death  the  heart  and  great  veins  are  much  distended 
with  blood,  the  arteries  comparatively  empty.  Even  after 
rigor  mortis  has  set  in,  the  right  heart  is  still  much  engorged. 

These  phenomena  are  the  result  of  the  operation  of  several 
causes.  The  increasingly  venous  blood  at  first  stimulates  the 
heart  probably  directly,  in  part  at  least,  but  later  has  the  con- 
trary effect.  The  nutrition  of  the  organ  suffers  from  the  de- 
graded blood,  from  which  it  must  needs  derive  its  supplies. 
The  cardio-inhibitory  center  probably  has  a  large  share  in  the 
slowing  of  the  heart,  if  not  also  in  quickening  it.  Whether 
the  accelerator  fibers  of  the  vagus  or  sympathetic  play  any 
part  is  uncertain.  The  increase  of  peripheral  resistance  caused 
by  the  action  of  the  vaso-motor  center  makes  it  more  difficult 
for  the  heart  to  empty  its  left  side  and  thus  receive  the  venous 
blood  as  it  pours  on.  At  the  same  time  the  deep  inspirations 
(when  the  chest  is  unopened)  favor  the  onflow  of  venous  blood; 


THE   RESPIRATORY   SYSTEM.  401 

and  in  any  case  the  whole  venous  system,  including  the  right 
heart,  tends  to  become  engorged  from  these  several  causes  act- 
ing together.  The  heart  gives  up  the  struggle,  unable  to  main- 
tain it,  but  not  so  long  as  it  can  beat  in  any  part. 

The  share  which  the  elasticity  of  the  arteries  takes  in 
forcing  on  the  blood  when  the  heart  ceases,  and  the  contraction 
of  the  muscular  coat  of  these  vessels,  especially  the  smaller, 
must  not  be  left  out  of  the  account  in  explaining  the  phenom- 
ena of  asphyxia  and  the  post-mortem  appearances. 

Pathological.— The  importance  of  being  practically  as  well 
as  theoretically  acquainted  with  the  facts  of  asphyxia  is  very 
great. 

The  appearance  of  the  heart  and  venous  system  gives  une- 
quivocal evidence  as  to  the  mode  of  death  in  any  case  of  as- 
phyxia; and  the  contrast  between  the  heart  of  an  animal  bled 
to  death,  or  that  has  died  of  a  lingering  disease,  and  one 
drowned,  hanged,  or  otherwise  asphyxiated,  is  extreme. 

We  strongly  recommend  the  student  to  asphyxiate  some 
small  mammal  placed  under  the  influence  of  an  anaesthetic, 
and  to  note  the  phenomena,  preferably  with  the  chest  opened ; 
and  to  follow  up  these  observations  by  others  after  the  onset  of 
rigor  mortis. 

PECULIAR   RESPIRATORY   MOVEMENTS. 

Though  at  first  sight  these  seem  so  different,  and  are  so  as 
regards  acts  of  expression,  yet  from  the  respiratory  point  of 
view  they  resemble  each  other  closely;  they  are  all  reflex,  and, 
of  course,  involuntary.  Many  of  them  have  a  common  pur- 
pose, either  the  better  to  ventilate  the  lungs,  to  clear  them  of 
foreign  bodies,  or  to  prevent  their  ingress. 

Coughing,  in  which  such  a  purpose  is  evident,  is  made  up  of 
several  expiratory  efforts  preceded  by  an  inspiratory  act.  The 
afferent  nerve  is  usually  the  vagus  or  laryngeal,  but  may  be  one 
or  more  of  several  others. 

The  glottis  presents  characteristic  appearances,  being  closed 
and  then  opened  suddenly,  the  mouth  being  kept  open. 

Coughing  is  often  induced  in  attempting  to  examine  the  ear 
with  instruments.     (Reflex  act). 

Laughing  is  very  similar  to  the  last,  so  far  as  the  behavior 
of  the  glottis  is  concerned,  though  it  usually  acts  more  rapidly, 
of  course.     Several  expirations  follow  a  deep  inspiration. 
26 


402  COMPARATIVE   PHYSIOLOGY. 

Crying  is  essentially  the  same  as  laughing,  hut  the  facial 
expression  is  different,  and  the  lachrymal  gland  functions  ex- 
cessively, though  with  some  persons  this  occurs  during  laughter 
also. 

Sobbing  is  made  up  of  a  series  of  inspirations,  in  which  the 
glottis  is  partially  closed,  followed  by  a  deep  expiration. 

Yawning  involves  a  deep-drawn,  slow  inspiration,  followed 
by  a  more  sudden  expiration,  with  a  well-known  depression  of 
the  lower  jaw  and  usually  stretching  movements. 

Sighing  is  much  like  the  preceding,  though  the  mouth  is  not 
opened  widely  if  at  all,  nor  do  the  stretching  movements  com- 
monly occur. 

Hiccough  is  produced  by  a  sudden  inspiratory  effort,  though 
fruitless,  inasmuch  as  the  glottis  is  suddenly  closed.  It  is 
spoken  of  as  spasm  of  the  diaphragm,  and  when  long  continued 
is  very  exhaustive. 

Sneezing  is  the  result  of  a  powerful  and  sudden  expiratory 
act  following  a  deep  inspiration,  the  mouth  being  usually  closed 
by  the  anterior  pillars  of  the  fauces  against  the  outgoing  cur- 
rent of  ah",  which  then  makes  its  exit  through  the  nose,  while 
the  glottis  is  forcibly  opened  after  sudden  closure.  It  will  be 
noticed  that  in  most  of  these  acts  the  glottis  is  momentarily 
closed,  which  is  never  the  case  in  mammals  during  quiet  res- 
piration. 

This  temporary  occlusion  of  the  respiratory  passages  per- 
mits of  a  higher  intrapulmonary  pressure,  which  is  very  effect- 
ive in  clearing  the  passages  of  excess  of  mucus,  etc.,  when  the 
glottis  is  suddenly  opened.  Though  the  acts  described  are  all 
involuntary,  they  may  most  of  them  be  imitated  and  thus 
studied  deliberately  by  the  student.  It  will  also  appear,  con- 
sidering the  many  ways  in  which  some  if  not  all  of  them  may 
be  brought  about,  that  if  the  medullary  center  is  responsible  for 
the  initiation  of  them  it  must  be  accessible  by  numberless  paths. 

Comparative. — Few  of  the  lower  animals  cough  with  the 
same  facility  as  man,  while  laughing  is  all  but  unknown,  cry- 
ing and  sobbing  rare,  though  the  whining  of  dogs  is  allied  to 
the  crying  of  human  beings. 

Sneezing  seems  to  be  voluntary  in  some  animals,  as  squir- 
rels, when  engaged  in  toilet  operations,  etc. 

Barking  is  voluntary,  and  in  mechanism  resembles  cough- 
ing, the  vocal  cords  being,  however,  more  definitely  employed, 
as  also  in  growling. 


THE   RESPIRATORY    SYSTEM.  403 

Balding,  neighing,  braying,  etc.,  are  made  up  of  long  ex- 
piratory acts,  preceded  by  one  or  more  inspirations.  The  vocal 
cords  are  also  rendered  tense. 


SPECIAL   CONSIDERATIONS. 

Pathological  and  Clinical.  —The  number  of  diseases  that  lessen 
the  amount  of  available  pulmonary  tissue,  or  hamper  the  move- 
ments of  the  chest,  are  many,  and  only  the  briefest  reference 
can  be  made  to  a  few  of  them. 

Inflammation  of  the  lungs  may  render  a  greater  or  less  por- 
tion of  one  or  both  lungs  solid;  inflammation  of  the  pleura 
(pleuritis,  pleurisy)  by  the  dryness,  pain,  etc.,  may  restrict  the 
thoracic  movements;  phthisis  may  solidify  or  excavate  the 
lungs,  or  by  pleuritic  inflammation  glue  the  costal  and  pulmo- 
nary pleural  surfaces  together;  bronchitis  may  clog  the  tubes 
and  other  air-passages  with  altered  secretions ;  emphysema  (dis- 
tention of  air-cells)  may  destroy  elasticity  of  parts  of  the  lung ; 
pneuma-thorax  from  rupture  of  the  lung-tissue  and  consequent 
accumulation  of  gases  in  the  pleural  cavity,  or  pleurisy  with 
effusion  render  one  lung  all  but  useless  from  pressure.  In  all 
such  cases  Nature  attempts  to  make  up  what  is  lost  in  amplitude 
by  increase  in  rapidity  of  the  respiratory  movements.  It  is 
interesting  to  note  too  how  the  other  lung,  in  diseased  condi- 
tions, if  it  remain  unaffected,  enlarges  to  compensate  for  the 
loss  on  the  opposite  side.  When  the  muscles  are  weak,  espe- 
cially if  there  be  hindrance  to  the  entrance  of  air  while  the 
thoracic  movements  are  marked,  there  may  be  bulging  inward 
of  the  intercostal  spaces. 

Normally,  this  would  also  occur,  as  the  intra-thoracic  press- 
ure is  less  than  the  atmospheric,  were  it  not  for  the  fact  that  the 
intercostal  muscles  when  contracting  have  a  certain  resisting 
power. 

The  imperfect  respiration  of  animals  when  dying,  permitting 
the  accumulation  of  carbonic  anhydride  with  its  soporific 
effects,  smooths  the  way  leading  to  the  end ;  so  that  there  may 
be  to  the  uninitiated  the  appearance  of  a  suffering  which  does 
not  exist,  consciousness  itself  being  either  wholly  or  partially 
absent.  The  dyspnoea  of  anaemic  animals,  whether  from  sud- 
den loss  of  blood  or  from  imperfect  renewal  of  the  haemo- 
globin, shows  that  this  substance  has  a  respiratory  function; 
while  in  forms  of  cardiac  disease  with  regurgitation,  etc.,  the 


404  COMPARATIVE   PHYSIOLOGY. 

blood  may  be  imperfectly  oxidized,  giving  rise  to  labored  res- 
piration. 

Personal  Observation. — As  hinted  from  time  to  time  during 
the  treatment  of  this  subject,  there  is  a  large  number  of  facts 
the  student  may  verify  for  himself. 

A  simple  way  of  proving  that  CO2  is  exhaled  is  to  breathe 
(blow)  into  a  vessel  containing  some  clear  solution  of  quick- 
lime (CaO),  the  turbidity  showing  that  an  insoluble  salt  of  lime 
(CaC03)  has  been  formed  by  the  addition  of  this  gas. 

The  functions  of  most  of  the  respiratory  muscles,  the  phe- 
nomena of  dyspnoea,  apncea  (by  a  series  of  long  breaths),  par- 
tial asphyxia  by  holding  the  breath,  and  many  other  experi- 
ments, simple  but  convincing,  will  occur  to  the  student  who  is 
willing  to  learn  in  this  way. 

The  observation  of  respiration  in  a  dreaming  animal  (dog) 
will  show  how  mental  occurrences  affect  the  respiratory  center 
in  the  absence  of  all  the  usual  outward  influences.  The  respira- 
tion of  the  domestic  animals,  and  of  the  frog,  turtle,  snake,  and 
fish,  is  easily  watched  if  these  cold-blooded  animals  be  placed 
for  observation  beneath  a  glass  vessel.  Their  study  will  teach 
how  manifold  are  the  ways  by  which  the  one  end  is  attained. 
Compare  the  tracings  of  Fig.  305. 

Evolution. — A  study  of  embryology  shows  that  the  respira- 
tory and  circulatory  systems  develop  together ;  that  the  vascu- 
lar system  functions  largely  as  a  respiratory  system  also  in  cer. 
tain  stages,  and  remains  such,  from  a  physiological  point  of 
view,  throughout  embryonic  life. 

The  changes  that  take  place  in  the  vascular  system — the 
heart,  especially — of  the  mammal  when  the  lungs  have  become 
functionally  active  at  birth,  show  how  one  set  of  organs  modi- 
fies the  other. 

When  one  considers,  in  addition  to  these  facts,  that  the 
digestive  as  well  as  the  vascular  and  respiratory  organs  are 
represented  in  one  group  of  structures  in  a  jelly-fish,  and  that 
the  lungs  of  the  mammal  are  derived  from  the  same  mesoblast 
as  gives  rise  to  the  digestive  and  circulatory  organs,  many  of 
the  relations  of  these  systems  in  the  highest  groups  of  animals 
become  intelligible ;  but  unless  there  be  descent  with  modifica- 
tion, these  facts,  clear  enough  from  an  evolutionary  standpoint, 
are  isolated  and  out  of  joint,  bound  together  by  no  common 
principle  that  satisfies  a  philosophical  biology. 

It  has  been  found  that  in  hunting-dogs  and  wild  rabbits  the 


THE  RESPIRATORY  SYSTEM.  405 

vagus  is  more  efficient  than  in  other  races  of  dogs  and  in  rab- 
bits kept  in  confinement ;  and  possibly  this  may  in  part  account 
for  the  greater  speed  and  especially  the  endurance  of  the 
former.  The  very  conformation  of  some  animals,  as  the  grey- 
hound, with  his  deep  chest  and  capacious  lungs,  indicates  an 
unusual  respiratory  capacity. 

The  law  of  habit  is  well  illustrated  in  the  case  of  divers,  who 
can  bear  deprivation  of  air  longer  than  those  unaccustomed  to 
such  submersion  in  water.  Greater  toleration  on  the  part  of 
the  respiratory  center  has  probably  much  to  do  with  the  case, 
though  doubtless  many  other  departures  from  the  normal  occur, 
either  independently  or  correlated  to  the  changes  in  the  respira- 
tory center.  Some  mammals,  like  the  whale,  can  long  remain 
under  water. 

Summary  of  the  Physiology  of  Respiration.— The  purpose 
of  respiration  in  all  animals  is  to  furnish  oxygen  for  the  tissues 
and  remove  the  carbonic  anhydride  they  produce,  which  in  all 
vertebrates  is  accomplished  by  the  exposure  of  the  blood  in 
capillaries  to  the  atmospheric  air,  either  free  or  dissolved  in 
water.  A  membrane  lined  with  cells  always  intervenes  between 
the  capillaries  and  the  air. 

The  air  may  be  pumped  in  and  out,  or  sucked  in  and  forced 
out. 

Respiration  in  the  Mammal.— The  air  enters  the  lungs, 
owing  to  the  enlargement  of  the  chest  in  three  directions  by  the 
action  of  certain  muscles.  It  leaves  the  lungs  because  of  their 
own  elastic  recoil  and  that  of  the  chest- wall  chiefly.  Inspiration 
is  active,  expiration  chiefly  passive. 

The  diaphragm  is  the  principal  muscle  of  respiration.  In 
some  animals  there  is  a  well-marked  facial  and  laryngeal  as 
well  as  thoracic  respiration.  Respiration  is  rhythmical,  con- 
sisting of  inspiration,  succeeded  without  appreciable  pause  by 
expiration,  the  latter  being  in  health  of  only  slightly  longer 
duration.  There  is  also  a  definite  relation  between  the  number 
of  respirations  and  of  heart-beats.  According  as  respiration  is 
normal,  hurried,  labored,  or  interrupted,  we  describe  it  as 
eupnoe,  hyperpnoea,  dyspnoea,  and  apnoea.  The  intra-thoracic 
pressure  is  never  equal  to  the  atmospheric — i.  e.,  it  is  always 
negative — except  in  forced  expiration  ;  and  the  lungs  are  never 
collapsed  so  long  as  the  chest  is  unopened.  The  expired  air 
diffei'S  from  that  inspired  in  being  of  the  temperature  of  the 
body,  saturated  with  moisture,  and  containing  about  4  to  5  per 


406  COMPARATIVE  PHYSIOLOGY. 

cent  less  oxygen  and  4  per  cent  more  carbonic  anhydride,  be- 
sides certain  indifferently  known  bodies,  the  result  of  tissue 
metabolism,  excreted  by  the  lungs. 

The  quantity  of  air  actually  moved  by  a  respiratory  act,  as 
compared  with  the  total  capacity  of  the  respiratory  organs,  is 
small  ;  hence  a  great  part  must  be  played  by  diffusion.  The 
portion  of  air  that  can  not  be  removed  from  the  lungs  by  any 
respiratory  effort  is  relatively  large. 

It  is  customary  to  distinguish  tidal,  complementary,  supple- 
mentary, and  residual  air. 

The  vital  capacity  is  estimated  by  the  quantity  of  air  the 
respiratory  organs  can  move,  and  is  very  variable. 

The  blood  is  the  respiratory  tissue,  through  the  mediation 
of  its  red  cells,  by  the  haemoglobin  they  contain.  This  sub- 
stance is  a  ferruginous  proteid,  capable  of  crystallization,  and 
assuming  under  chemical  treatment  many  modifications.  When 
it  contains  all  the  oxygen  it  can  retain,  it  is  said  to  be  saturated 
and  is  called  oxy-haemoglobin,  in  which  form  it  exists  (with 
some  reduced  haemoglobin)  in  arterial  blood,  and  to  a  lesser 
extent  in  venous  blood,  which  differs  from  arterial  in  the  rela- 
tive proportions  of  haemoglobin  (reduced)  it  contains,  as  viewed 
from  the  respiratory  standpoint. 

Oxy-haemoglobin  does  not  assume  or  part  with  its  oxygen, 
according  to  the  Henry -Dalton  law  of  pressures,  nor  is  this  gas 
in  a  state  of  ordinary  chemical  combination.  It  is  found  that 
the  oxygen  tension  of  the  blood  is  lower  and  that  of  carbonic 
anhydride  higher  than  in  the  air  of  the  alveoli  of  the  lungs, 
while  the  same  may  be  said  of  the  tissues  and  the  blood  re- 
spectively.    This  has  been,  however,  recently  again  denied. 

Respiration  is  a  vital  process,  though  certain  physical  con- 
ditions (temperature  and  pressure)  must  be  rigidly  maintained 
in  order  that  the  gaseous  interchanges  shall  take  place.  Res- 
piration is  always  fundamentally  bound  up  with  the  metabo- 
lism of  the  tissues  themselves.  All  animal  cells,  whether  they 
exist  as  unicellular  animals  (Amoeba)  or  as  the  components  of 
complex  organs,  use  np  oxygen  and  produce  carbonic  dioxide. 
Respiratory  organs,  usually  so  called,  and  the  respiratory  tissue 
par  excellence  (the  blood)  are  only  supplementary  mechanisms 
to  facilitate  tissue  respiration.  Carbonic  anhydride  exists  in 
blood  probably  in  combination  with  sodium  salts,  though  the 
whole  matter  is  very  obscure. 

Respiration,  like  all  the  other  functions  of  the  body,  is  con- 


THE  RESPIRATORY  SYSTEM.  4Q7 

trolled  by  the  central  nervous  system  through  nerves.  The 
medulla  oblongata  is  chiefly  concerned,  and  especially  one 
small  part  of  it  known  as  the  respiratory  center.  It  is  possible, 
even  probable,  that  there  are  subordinate  centers  in  the  cord, 
which,  under  peculiar  circumstances,  assume  importance  ;  but 
how  far  they  act  in  concert  with  the  medullary  center,  or 
whether  they  act  at  all  when  normal  conditions  prevail,  is  an 
open  question. 

The  vagus  is  the  principal  afferent  respiratory  nerve.  The 
efferent  nerves  are  the  phrenies.  intercostals,  and  others  supply- 
ing the  various  muscles  used  in  moving  the  chest-walls,  etc. 

The  respiratory  center  is  automatic,  but  its  action  is  sus- 
ceptible of  modification  through  afferent  influences  taking  a 
variety  of  paths,  the  principal  of  which  is  along  the  vagi  nerves. 
The  respiratory,  vaso-motor,  and  cardio-inhibitory  centers  seem 
to  act  somewhat  in  concert. 

Blood-pressure  is  being  constantly  modified  by  the  respira- 
tory act,  rising  with  inspiration  and  sinking  with  expiration. 
In  some  animals  the  heart-beat  also  varies  with  these  phases  of 
respiration,  becoming  slow  and  irregular  during  expiration. 
Into  the  causation  of  these  changes  both  mechanical  and  nerv- 
ous factors  enter,  and  make  a  very  complex  mesh,  which  we 
can  at  present  but  imperfectly  unravel.  When  the  access  of 
air  to  the  tissues  is  prevented,  a  series  of  stages  of  respiratory 
activity  and  decline,  accompanied  by  pronounced  changes  in 
the  vascular  system,  are  passed  through,  known  as  asphyxia. 

Three  stages  are  distinguishable  :  one  of  dyspnoea,  one  of 
convulsions,  and  one  of  exhaustion — while  at  the  same  time 
there  is  a  rise  of  blood-pressure  during  the  first  two,  and  a 
decline  during  the  third,  accompanied  by  marked  alterations  in 
the  cardiac  rhythm. 


PROTECTIVE    AND   EXCRETORY  FUNCTIONS 
OF   THE   SKIN. 


As  lias  been  intimated  from  time  to  time,  thus  far,  as  a 
result  of  the  metabolism  of  the  tissues,  certain  products  require 
constant  removal  from  the  blood  to  prevent  poisonous  effects. 
These  substances  are  in  all  probability  much  more  numerous 
than  physiological  chemistry  has  as  yet  distinctly  recognized 
or,  at  all  events,  isolated.  Quantitatively  considered,  the  most 
important  are  carbonic  anhydride,  water,  urea,  and,  of  less  im- 
portance, perhaps,  certain  salts. 

In  many  invertebrates  and  in  all  vertebrates  several  organs 
take  part  in  this  work  of  elimination  of  waste  products  or  puri- 
fication of  the  blood,  one  set  of  which — the  respiratory — we 
have  just  studied  ;  and  we  now  continue  the  consideration  of 
the  subject  of  excretion,  this  term  being  reserved  for  the  pro- 
cess of  separating  harmful  products  from  the  blood  and  dis- 
charging them  from  the  body. 

We  strongly  recommend  the  student  to  make  the  study  of 
excretion  comparative  in  the  sense  of  noting  how  one  organ 
engaged  in  the  process  supplements  another.  A  clear  under- 
standing of  this  relation  even  to  details  makes  the  practice  of 
medicine  more  scientific  and  practically  effective,  and  gives 
physiology  greater  breadth. 

The  skin  has  a  triple  function  :  it  is  protective,  excretory, 
sensory,  and,  we  may  add,  nutritive  (absorptive)  and  respira- 
tory, especially  in  some  groups  of  animals. 

As  a  sensory  organ,  the  skin  will  receive  attention  later. 

Protective  Function  of  the  Skin. — Comparative. — Among 
many  groups  of  invertebrates  the  principal  use  of  the  exterior 
covering  of  the  body  is  manifestly  protection.  Among  these 
forms,  an  internal  skeleton  being  absent,  the  exo-skeleton  is 
developed  externally,  and  serves  not  only  for  protection,  but 
for  the  attachment  of  muscles,  as  seen  in  crustaceans  and  in- 


PROTECTIVE   FUNCTION  OP  THE  SKIN. 


409 


sects.  But  this  part  of  the  subject  is  too  large  for  detailed 
treatment  in  such  a  work  as  this.  Turning  to  the  vertebrates, 
we  see  scales,  bony  plates,  feathers,  spines,  hair,  etc.,  most  of 
them  to  be  regarded  as  modifications  of  the  epidermis,  always 
useful,  and  frequently  also  ornamental. 

Primitive  man  was  probably  much  more  hirsute  than  his 
modern  representative  ;  and,  though  the  human  subject  is  at 
present  provided  with  a  skin  in  which  protective  functions  are 
at  their  lowest,  still  the  epidermis  does  serve  such  a  purpose,  as 

all  have  some  time  realized  when 
it  has  been  accidentally  removed 
by  blistering,  etc. 

Taking  the  structure  of  the 
skin  of  man  as  representing  that 
of  mammals  generally,  certain 
points  claim  attention  from  the 
physiologist.  Its  elasticity,  the 
failure  of  which  in  old  age  ac- 
counts for  wrinkles;  its  epider- 


E> 


■jrftSfea^y    •"?}..-  -*3?' ,v^ 


Fig.  311. 


Fig.  312. 


Fig.  311.— Sudoriparous  gland?.  1x20.  (Af ter  Sappey.)  1.  1.  epidermis;  2.  2,  mucous 
layer;  3,  3,  papillae;  4.  4,  derma;  5.  5,  subcutaneous  areola  tissue;  6.  0.  6,  6,  sudo- 
riparous glands;  7  7,  adipose  vesicles;  8,  8,  excretory  ducts  in  derma;  9,  9,  excre- 
tory ducts  divided. 

Fig.  312. — Portion  of  skin  of  palm  of  hand  about  one-half  an  inch  (127  mm.)  square. 
1x4.  (After  Sappey.)  1, 1, 1, 1,  openings  of  sudoriferous  ducts;  2,  2,  2,  2,  grooves 
between  papillse  of  skin. 

mal  covering,  made  up  of  numerous  layers  of  cells;  its  coiled 
and  spirally  twisted  sudoriferous  glands,  permitting  of  move- 
ments of  the  skin  without  harm  to  these  structures;  its  hair- 
follicles  and  associated  sebaceous  glands,  the  fatty  secretion  of 
which  keeps  the  hair  and  the  skin  generally  soft  and  pliable. 


410 


COMPARATIVE   PHYSIOLOGY. 


Tlie  muscles  of  the  skin,  which  either  move  it  as  a  whole  or 
erect  individual  hairs,  play  an  important  part  in.  modifying"  ex- 
pression, well  seen  in  the  whole  canine  tribe  and  many  others. 

There  are  several 
modifications  of  the 
sebaceous  glands  that 
furnish  highly  odor- 
iferous secretions  as 
in  the  civet  cat,  the 
skunk,  the  musk- 
deer,  and  many  low- 
er vertebrates.  In 
some,  these  are  pro- 
tective (skunk)  ;  in 
others,  though  they 
may  not  be  agreeable 
to  the  senses  of  man, 
they  are  doubtless  at- 
tractive to  the  fe- 
males of  the  same 
tribe,  and  are  to  be 
regarded  as  impor- 
tant in  "  sexual  se- 
lection," being  often 
confined  to  the  males 
alone. 

Ear-wax  and  the 
Meibomian  secretion 
are  the  work  of 
modified  sebaceous 
glands  ;  as  also  the 
oil-glands  so  highly 
developed  in  birds, 
especially  aquatic 
forms,  and  of  which 
these  creatures  make 
great  use  in  preserv- 

Fig.  313.— Hair  and  hair-follicle  (after  Sappey).    1,  root  ing     their      feathers 

of  hair;  2,  bulb  of  hair;  3,  internal  root-sheath;  5,  from  wettin0- 
membrane  of  hair-follicle;  6,  external  membrane  of  & 

follicle;  7,  7,  muscular  bands  attached  to  follicle;  In    our    ClomOSUC 

8,  8,  extremities  of  bands  passing  to  skin;  9,  com-  .  ™„,T  ac! 

pound  sebaceous  Kiand,  with  duct  (10)  opening  into  animals  we  may  es- 

W^gd$^^'*amnB*"i   pecially  notice  a  cu 


THE  EXCRETORY  FUNCTION   OF  THE   SKIN.        4H 


taneous  gland  in  the  pig,  placed  at  the  posterior  inner  aspect  of 
the  knee  and  of  considerable  size. 

In  the  sheep,  the  interungulate 
gland  is  an  inversion  of  the  integu- 
ment forming  an  elongated  sac, 
which  is  supplied  with  secreting 
structures  analogous  to  the  seba- 
ceous glands.  The  importance  of 
protective  structures  of  this  kind 
in  such  situations  is  obvious. 

THE    EXCRETORY    FUNCTION 
OF  THE    SKIN. 

The    quantity    of      matter    dis-    Fig-   314.  —  Interungulate  gland  of 
,  ,    .,  i,i         i  •       •     i  sheep  (Chauveau).    «,  inner  as- 

charged  through  the  skm  is  large  pectof  first  phalanx;  b,  hoof  or 
-greater  in  man  than  by  the  lungs  Se  Viteffi"1^  *tad;  * 
(about    as    7    to  11),   though    the 

amount  is  very  variable,  depending  on  the  degree  of  activ- 
ity of  other  related  excreting  organs,  as  the  lungs  and  kidneys, 
and  largely  upon  the  temperature  as  a  physical  condition;  and 
so  in  other  animals. 

When  the  watery  vapor  is  carried  off,  before  it  can  condense, 
the  perspiration  is  said  to  be  insensible ;  when  small  droplets 
become  visible,  sensible.  As  to  whether  the  one  or  the  other  is 
predominant  will,  of  course,  depend  on  the  rapidity  of  renewal 
of  the  air,  its  humidity,  and  its  temperature.  Apart  from  the 
temperature,  the  amount  of  sweat  is  influenced  by  the  quality 
and  quantity  of  food  and,  especially  of  drink  taken,  the  amount 
of  exercise,  and  psychic  conditions ;  not  to  speak  of  the  effect  of 
drugs,  poisons,  or  disease. 

Perspiration  in  man  is  a  clear  fluid,  mostly  colorless,  with 
a  characteristic  odor,  devoid  of  morphological  elements  (ex- 
cept epidermal  scales),  and  alkaline  in  reaction.  It  may  be 
acid  from  the  admixture  of  the  secretion  of  the  sebaceous 
glands. 

Its  solids  (less  than  2  per  cent)  consist  of  sodium  salts, 
mostly  chlorides,  cholesterin,  neutral  fats,  and  traces  of  urea. 
The  acids  of  the  sweat  belong  to  the  fatty  series  (acetic,  butyric, 
formic,  propionic,  caprylic,  cax^roic,  etc.). 

Pathological. — The  sweat  may  contain  blood,  proteids,  abun- 
dance of  urea  (in  cholera),  uric  acid,  oxalates,  sugar,  lactic  acid, 


412  COMPARATIVE  PHYSIOLOGY. 

bile,  indigo,  and  other  pigments.  Many  medicines  are  elimi- 
nated in  part  through  the  skin. 

Respiration  by  the  Skin.— Comparative.— In  i-eptiles  and 
batrachians,  with  smooth,  moist  skin,  the  respiratory  functions 
of  this  organ  are  of  great  importance ;  hence  these  animals  can 
live  long  under  water. 

It  is  estimated  that  in  the  frog  the  greater  part  of  the  car- 
bonic anhydride  of  the  body -waste  is  eliminated  by  the  skin. 
Certainly  frogs  can  live  for  days  immersed  in  a  tank  supplied 
with  running  water  ;  and  it  is  a  significant  fact  that  in  this 
animal  the  vessel  that  gives  rise  to  the  pulmonary  artery  sup- 
plies also  a  cutaneous  branch. 

The  respiratory  capacity  of  the  skin  in  man  and  most 
mammals  is  comparatively  small  under  ordinary  circum- 
stances. The  amount  of  carbonic  anhydride  thus  eliminated 
in  twenty-four  hours  in  man  is  estimated  at  not  more  than  10 
grammes.  It  varies  greatly,  however,  with  temperature,  exer- 
cise, etc. 

The  skin  is  highly  vascular  in  mammals,  and  its  importance 
as  a  heat  regulator  is  thus  very  great. 

When  an  animal  is  varnished  over,  its  temperature  rapidly 
falls,  though  heat  production  is  in  excess.  From  the  fact  that 
life  may  be  prolonged  by  diminishing  loss  of  heat  through 
wrapping  up  the  animal  in  cotton-wool,  it  is  inferred  that 
depression  of  the  temperature  is,  at  all  events,  one  of  the  causes 
of  death.  Though  the  subject  is  obscure,  it  is  likely  that  the 
retention  of  poisonous  products  so  acts  as  to  derange  metabo- 
lism, as  well  as  poison  directly,  which  might  thus  lead  to  the 
disorganization  of  the  machinery  of  life  to  the  point  of  disrup- 
tion or  death.  It  is  also  possible  that  the  reduction  of  the  tem- 
perature from  dilatation  of  the  cutaneous  vessels  may  be  so 
great  that  the  animal  is  cooled  below  that  point  at  which  the 
vital  functions  can  continue. 

THE   EXCRETION   OF   PERSPIRATION. 

In  secretion  in  the  wider  sense  we  find  usually  certain  nerv- 
ous and  vascular  effects  associated.  The  vessels  supplying  the 
gland  are  dilated  during  the  most  active  phase,  and  at  the  same 
time  nervous  impulses  are  conveyed  to  the  secreting  cells  which 
stimulate  them  to  action.  There  is  a  certain  proportion  of 
water  given  off  by  transpiration  ;    but  the  sweat,  as  a  whole, 


THE  EXCRETION   OP   PERSPIRATION.  413 

even  the  major  part  of  the  water,  is  a  genuine  secretion,  the 
result  of  the  metaholism  of  the  cells. 

From  experiments  it  is  clear  that  nervous  influences  alone, 
in  the  absence  of  any  vascular  changes,  or  in  the  total  depriva- 
tion of  blood,  suffice  to  induce  the  secretion  of  perspiration.  If 
the  central  stump  of  the  divided  sciatic  be  stimulated,  sweating 
of  the  other  limbs  follows,  showing  that  perspiration  may  be  a 
reflex  act.  It  is  found  that  stimulation  of  the  peripheral  end  of 
the  divided  cervical  sympathetic  leads  to  sweating  on  the  cor- 
responding side  of  tbe  face. 

Sweating  during  dyspnoea  and  from  fear,  when  the  cutane- 
ous surfaces  are  pale,  as  well  as  in  the  dying  animal,  shows  also 
the  independent  influence  over  the  sudorific  glands  of  the  nerv- 
ous system.  Heat  induces  sweating  by  acting  both  reflexly  and 
directly  on  the  sweat-centers  we  may  suppose.  Unilateral 
sweating  is  known  as  a  pathological  as  well  as  experimental 
phenomenon.  Perspiration  may  be  either  increased  or  dimin- 
ished in  paralyzed  limbs,  according  to  circumstances.  It  is 
possible  that  there  is  a  paralytic  secretion  of  sweat  as  of  saliva. 
The  subject  is  very  intricate,  and  will  be  referred  to  again  on 
account  of  the  light  it  throws  on  metabolic  pi'ocesses  generally. 

Absorption  by  the  skin  in  man  and  other  mammals  is,  under 
natural  conditions  probably  very  slight,  as  would  be  expected 
when  it  is  borne  in  mind  that  the  true  skin  is  covered  by  sev- 
eral layers  of  cells,  the  outer  of  which  are  hardened. 

Ointments  may  unquestionably  be  forced  in  by  rubbing  ; 
and  perhaps  absorption  may  take  place  when  an  animal's  tis- 
sues are  starving,  and  food  can  not  be  made  available  through 
the  usual  channels.  It  is  certain  that  abraded  surfaces  are  a 
source  of  danger,  from  affording  a  means  of  entrance  for  dis- 
ease-producing substances  or  for  germs. 

Comparative. — It  is  usually  stated  in  works  on  physiology 
that  the  horse  sweats  profusely,  the  ox  less  so ;  the  pig  in  the 
snout:  and  the  dog,  cat,  rabbit,  rat,  and  mouse,  either  not  at  all 
or  in  the  feet  (between  the  toes)  only.  That  a  closer  observa- 
tion of  these  animals  will  convince  any  one  that  the  latter 
statements  are  not  strictly  correct,  we  have  no  doubt.  These 
animals,  it  is  true,  do  not  perspire  sensibly  to  any  great  extent ; 
but  to  maintain  that  their  skin  has  no  excretory  function  is  an 
error. 

Summary.— The  skin  of  the  mammal  has  protective,  sensory, 
respiratory,  and  excretory  functions.     The  respiratory  are  in- 


414  COMPARATIVE   PHYSIOLOGY. 

significant  under  ordinary  circumstances  in  this  group,  though 
well  marked  in  reptiles  and  especially  in  hatrachians  (frog, 
menobranchus).  Sweating  is  probably  dependent  on  the  action 
of  centers  situated  in  the  brain  and  spinal  cord,  through  nerves 
that  run  generally  in  sympathetic  tracts  during  some  part  of 
their  course.  While  the  function  of  sweating  may  go  on  inde- 
pendently of  abundant  blood-supply,  it  is  usually  associated 
with  increased  vascularity. 

Sweat  contains  a  very  small  quantity  of  solids,  is  alkaline  in 
reaction  when  pure,  but  liable  to  be  acid  from  the  admixture  of 
sebaceous  matter  that  has  undergone  decomposition.  Sebum 
consists  chiefly  of  olein,  palmitin,  soaps,  cholesterin,  and  ex- 
tractives of  little  known  composition.  The  salty  taste  of  the 
perspiration  is  due  chiefly  to  sodium  chloride,  and  its  smell  to 
volatile  fatty  acids ;  especially  is  this  so  of  the  sweat  of  certain 
parts  of  the  body  of  man  and  other  mammals. 

The  functional  activity  of  the  skin  varies  with  the  tempera- 
ture, moisture,  etc.,  of  the  air  and  certain  internal  conditions; 
especially  is  it  important  to  remember  that  it  is  one  of  a  series 
of  excretory  organs  which  act  in  harmony  to  eliminate  the 
waste  of  the  body,  so  that  when  one  functions  more  the  other 
may  and  usually  does  function  less. 

The  protective  function  of  the  skin  and  its  modified  epithe- 
lium (hair,  horns,  nails,  feathers,  etc.)  is  in  man  slight,  but  very 
important  in  many  other  vertebrates,  among  which  provision 
against  undue  loss  of  temperature  is  one  of  the  most  constantly 
operative,  and  enables  a  vast  number  of  groups  of  animals  to 
adapt  successfully  to  their  varying  surroundings. 


EXCEETION   BY   THE   KIDNEY. 


The  kidney  in  man  and  other  mammals  may  be  described  as 
a  very  complex  arrangement  of  tubes  lined  with  many  differ- 
ent forms  of  secreting-  cells,  surrounded  by  a  great  mesh  work  of 
capillaries,  bound  together  by  connective  tissue,  the  quantity 


Fig.  315.— Vertical  longitudinal  section  of  horse's  kidney  (Chauveau).  a,  cortical  por- 
tion; b,  medullary  portion;  c,  peripheral  portion  of  latter;  d,  interior  of  pelvis; 
d',  d',  arms  of  pelvis;  e,  border  of  crest;  /,  infundibulum;  g,  ureter. 

varying  with  the  animal,  and  the  whole  inclosed  in  a  capsule. 
The  organ  is  well  supplied  with  lymphatics  and  nerves. 
Though  the  tubes  are  so  complex,  the  kidney  may  be  divided 
into  zones  which  contain  mostly  but  one  kind  of  tubule. 

Among  vertebrates,  till  the  reptiles  are  reached,  the  kidney 
is  a  persistent  Wolffian  body,  hence  its  more  simple  form. 

In  most  fishes  the  kidney  is  a  very  elongated  organ,  though 


416 


COMPARATIVE  PHYSIOLOGY. 


T?,n  sip.  stricture  of  kidnev  (after  Landois).  I.  Blood-vessels  and  tubes  (semi-dia- 
SimS  A  Cap  Ss  of  cortical  substance.  B..  Capillaries  of  medu  lary 
gSSESS?" 1, ^impenetrating  Malpighian  body;  2,  van  _.„£ rom  a  Ma  pig- 
hem  hortv  It  arterioles  rectee:  C,  venm  rectse  7,  F,  interlobular  veins,  ^.stciiaie 
ve  ns  )  /  cansu  «•«  of  Mullur  X,  A',  convoluted  tubes;  T,  T,  T,  tubes  of  Henle; 
N  NNN  ■  diiimui.i.-utiiiK  tubes  0,  0,  straight  tubes;  O,  opening  into  pelvis  of 
kidney  '  II.  Malpighian  body.    A,  artery;  ti,  vein;  0,  capilfanes;  if,  epithelium 


EXCRETION  BY  THE  KIDNEY. 


41? 


of  capsule;  H,  beginning  of  convoluted  tube.  III.  Rodded  cells  from  convoluted 
tube.  1,  view  from  surface;  2,  side  view  {G,  granular  zone).  IV.  Cells  lining 
tubes  of  Henle.  V.  Cells  lining  communicating  tubes.  VI.  Section  of  straight 
tube. 

in  the  lowest  it  consists  of  little  more  than  tubules,  coiling  but 
slightly,  ending  by  one  extremity  in  a  glomerulus  and  by  the 


Fig.  317 


•  317.— Blood-vessels  of  MalpigTuan  bodies  and  convoluted  tubes  of  kidney  (after 
Sappey).  1,  1,  Malpighian  bodies  surrounded  by  capsules;  2.  2,  2.  convoluted 
tubes  connected  with  Malpighian  bodies;  3.  artery  branching  to  go  to  Malpighian 
bodies;  4,  4,  4.  branches  of  artery;  6,  6,  Malpighian  bodies  from"  which  a  portion 
of  capsules  has  been  removed;  7.  7,  7,  vessels  passing  out  of  Malpighian  bodies;  8. 
vessel,  branches  of  which  (9)  pass  to  capillary  plexus  (10). 

27 


418 


COMPARATIVE   PHYSIOLOGY. 


other  opening-  into  a  long-  common  efferent  tube  or  duct.  The 
glomerulus  is,  however,  peculiar  to  the  vertebrate  kidney. 
The  graded  complexity  in  arrangement,  etc.,  of  the  tubes  is 
represented  well  in  the  figure  below.     It  is  a  significant  fact 


Pig.  318.— Diagrammatic  representation  of  distribution  of  tubules  of  kidney  (after 
Huxley).  0,  cortical  region;  B.  boundary  zone,  containing  large  part  of  Henle's  , 
loops;  P,  papillary  zone,  in  which  are  the  main  outflow  tubules. 

that  the  kidney  of  the  human  subject  is  lobulated  in  the  embryo, 
which  condition  is  persistent  in  some  mammals  (ruminants,  etc.). 
As  the  lungs  are  the  organs  employed  especially  for  the 
elimination  of  carbonic  anhydride,  so  the  kidneys  are  above 
all  others  the  excretors  of  the  nitrogenous  waste  products  of  the 
body  chiefly  in  the  form  of  uric  acid  or  urea.  Before  treating  of 
secretion  by  the  kidney  it  will  be  well  to  examine  into  the  phys- 
ical and  chemical  properties  of  urine  with  some  detail,  especially 
on  account  of  its  great  importance  in  the  diagnosis  of  disease. 


EXCRETION  BY  THE  KIDNEY. 


419 


URINE     CONSIDERED    PHYSICALLY    AND    CHEMI- 
CALLY. 

Urine  is  naturally  a  fluid  of  very  variable  composition,  espe- 
cially regarded  quantitatively — a  fact  to  be  borne  in  mind  in 
considering'  all  statements  of  the  constitution  of  this  fluid. 

Specific  Gravity. — Urine  must  needs  be  heavier  than  water, 
on  account  of  the  large  variety  of  solids  it  contains.  The  aver- 
age specific  gravity  of  the  urine  for  the  twenty -four  hours  is  in 
man  1015  to  1020  ;  in  the  horse,  1030  to  1060  ;  in  the  ox,  1020 
to  1030 ;  in  the  sheep  and  goat,  1005  to  1015 ;  in  the  pig,  1010  to 
1015 ;  in  the  dog,  1030  to  1050.  It  is  lowest  in  the  morning  and 
varies  greatly  with  the  quantity  and  kind  of  food  eaten,  the  ac- 
tivity of  the  lungs  and  especially  of  the  skin,  etc. 

Color. — Some  shade  of  yellow,  which  is  also  very  variable, 
being  increased  in  depth  either  by  the  presence  of  an  excess  of 
pigment  or  a  diminution  of  water.  In  herbivora  the  urine  is 
turbid,  and  may  darken  on  exposure  to  the  air. 

The  reaction  of  human  urine  is  acid,  owing  to  acid  salts, 
especially  acid  sodium  phosphate  (NafLPQ,).  In  the  carnivora  it 
is  strongly  acid ;  in  the  herbivora,  alkaline.  The  reaction  of  urine 
depends  largely  though  not  wholly  on  the  character  of  the  food. 

Quantity. — This  is,  of  course,  like  the  specific  gravity,  highly 
variable,  and  frequently  they  run  parallel  with  each  other. 

The  following  tabular  statement  will  prove  useful  for  refer- 
ence: 

Composition  of  the  Urine  (Boussingault). 


Horse.* 

Cow.t 

Pigt 

Urea 

31-0 
4-7 

20-1 

15-5 
4-2 

10-8 
1-2 
0-7 
1-0 

o-o 

910-0 

18-5 

16-5 

17-2 

16-1 

4-7 

06 

3-6 

1-5 

trace. 

o-o 

921-3 

4-9 

Potassium  hippurate 

o-o 

Alkaline  lactates.    

Potassium  bicarbonate 

10-7 

Magnesium  carbonate 

0-9 

Calcium  carbonate 

Potassium  sulphate 

20 

Sodium  chloride 

1-3 

01 

1-0 

Water  and  substances  undetermined. 

979-1 

Total 

1000-0 

1000-0 

1000-0 

*  Diet  of  clover,  grass,  and  oats. 
X  Diet  of  potatoes,  cooked. 


f  Diet  of  hay  and  potatoes. 


420  COMPARATIVE  PHYSIOLOGY. 

Nitrogenous  Crystalline  Bodies.— These  are  the  derivatives 
of  the  metabolism  of  the  body,  and  not  in  any  appreciable  de- 
gree drawn  froni  the  food  itself.  Besides  urea,  and  of  much  less 
importance,  occurring  in  small  quantities,  are  bodies  that  may 
be  regarded  as  less  oxidized  forms  of  nitrogenous  metabolism, 
such  as  creatinin,  xanthin,  hypoxanthin  (sarkin),  hippuric  acid, 

ammonium  oxalurate,  and  urea,  CO  >  tvttt2  The  latter  was 
first  prepared  artificially  from  ammonium  cyanate,  |jtt  I  O, 

with  which  it  is  isomeric.  The  quantity  of  urea  is  generally 
in  inverse  proportion  to  that  of  hippuric  acid,  and  varies  much 
with  the  diet  in  the  herbivora.  The  richer  in  proteids  the  diet, 
as  when  oats  are  fed,  the  greater  the  quantity  of  urea.  In  the 
horse  this  proportion  varies  with  the  ordinary  diet  between 
2*5  and  4"0  per  cent. 

When  air  has  free  access  to  urine  for  some  time  in  a  warm 
room,  the  urea  becomes  ammonium  carbonate  by  hydration, 
probably  owing  to  the  influence  of  micro-organisms,  thus: 
CO  (NH2)o+  2  H,0  =  (NH4)2  C03;  hence  the  strong  ammoniacal 
smell  of  old  urine,  urinals,  etc. 

Uric  acid  (C5H4N4O3)  occurs  sparingly  (see  table),  combined 
with  sodium  and  ammonium  chiefly  as  acid  salts. 

Non-nitrogenous  Organic  Bodies.— A  series  of  well-known 
aromatic  bodies  occurs  in  urine,  especially  in  that  of  the  horse, 
cow,  etc.  These  are  phenol,  cresol,  pyrocatechin,  etc.,  which 
occur  not  free,  but  united  with  sulphuric  acid. 

Inorganic  Salts. — These  are  mostly  in  simple  solution,  in 
urine,  and  not  as  in  some  other  fluids  of  the  body,  united  with 
proteid  bodies.  The  salts  are  chlorides,  phosphates,  sulphates, 
nitrates,  and  carbonates  ;  the  bases  being  sodium,  potassium, 
calcium,  magnesium.  The  imosphates  are  to  be  traced  to  the 
food,  to  the  phosphorus  of  proteids,  and  to  phosphorized  fats 
(lecithin).  The  sulphates  are  derived  from  those  of  the  food 
and  from  the  sulphur  of  the  proteids  of  the  body.  The  greater 
part  of  the  carbonates  is  supplied  directly  by  the  food.  In  the 
horse  the  salts  of  potassium  and  calcium  (CaCOa),  are  abundant; 
while  in  the  dog  magnesium  and  calcium  salts  abound  as  sul- 
phates and  phosphates. 

Doubtless  many  bodies  appear  either  regularly  or  occasion- 
ally in  urine  that  have  so  far  escaped  detection,  which  are,  like 
the  poisonous  exhalations  of  the  lungs,  not  the  less  important 
because  unknown  to  science. 


EXCRETION  BY  THE   KIDNEY.  421 

Abnormal  Urine. — There  is  not  a  substance  in  the  urine  that 
does  not  vary  under  disease,  while  the  possible  additions  act- 
ually known  are  legion.  These  may  be  derived  either  from 
the  blood  or  from  the  kidneys  and  other  parts  of  the  urinary 
tract.  The  kidneys  seem  to  take  upon  themselves  more  readily 
than  any  other  organ  the  duty  of  eliminating  foreign  matters 
from  the  body.  But  this  aspect  of  the  subject  is  too  wide  for 
detailed  consideration  in  this  work. 

The  student  of  medicine  should  be  thoroughly  familiar  with 
the  urine  in  its  normal  condition  before  he  enters  upon  the 
examination  of  the  variations  produced  by  disease.  This  is  not 
difficult,  and  much  of  it  may  be  carried  out  with  but  a  meager 
supply  of  apparatus.  For  this  purpose,  however,  we  recom- 
mend some  work  devoted  to  the  chemical  and  microscopic  study 
of  the  urine. 

It  greatly  assists  to  remember  a  few  points  in  regard  to  solu- 
bilities. From  a  physiological  point  of  view,  the  urine  and  its 
variations,  as  the  result  of  changes  in  the  organism,  may  be  ob- 
served with  advantage  in  one's  own  person — eg.,  the  influence 
of  food  and  drink,  temperature,  emotions,  etc. 

Comparative.  —  In  fishes,  reptiles,  and  birds,  uric  acid  re- 
places urea,  and  is  very  abundant.  In  these  animals  most  of 
this  substance  is  white.     The  urine  is  passed  with  the  fasces. 

In  certain  groups  of  invertebrates  uric  acid  seems  to  be  a 
normal  excretion. 

THE    SECRETION   OF   URINE. 

By  means  of  apparatus  adapted  to  register  the  changes  of 
volume  the  kidney  undergoes,  it  is  found  that  this  organ  not 
only  responds  to  every  general  change  in  blood-pressure,  but 
to  each  heart-beat — that  is,  its  volume  varies  momentarily. 
This  shows  how  sensitive  it  is  to  variations  in  blood-pressure. 

Theories  regarding  the  secretion  of  urine  may  be  divided 
into  those  that  are  almost  wholly  physical,  partly  physical, 
and  purely  secretory:  1.  To  the  first  class  belongs  that  of 
Ludwig,  which  teaches  that  very  dilute  urine  is  separated  from 
the  blood  in  the  glomeruli,  and  by  a  process  of  osmosis  and 
absorption  of  water  by  the  tubular  capillaries  is  gradually 
concentrated  to  the  normal.  2.  As  an  example  of  the  second 
class  is  that  of  Bowman,  who  maintained  that  the  greater  part 
of  the  water  and  some  of  the  more  soluble  and  diffusible  salts 


422  COMPARATIVE  PHYSIOLOGY. 

are  separated  by  the  glomeruli  but  the  characteristic  constitu- 
ents of  the  urine  by  the  epithelium  of  the  renal  tubules.  3.  As 
an  example  of  the  third  is  the  theory  of  Heidenhain,  who  attrib- 
uted little  to  blood-pressure  in  itself,  and  much,  if  not  the 
whole,  to  the  secreting  activity  of  the  epithelium  of  the  tubules 
more  particularly.     This  physiologist  showed  that  while  liga- 


BLOOD     PRESSURE    CURVE 


VVWvVWA^ 


Fig.  319. — Blood-pressure  curve  and  curve  of  the  volume  of  the  kidney;  T,  time- 
curve,  intervals  indicate  a  quarter  of  a  minute;  A,  abscissa  (Stirling,  after  Koy). 

ture  of  a  vein  raised  the  blood-pressure  within  a  glomerulus,  it 
was  not  followed  by  any  increase  in  the  quantity  of  the  secretion, 
but  by  its  actual  arrest.  He  also  showed  that  injection  of  a  col- 
ored substance  (sodium  sulphindigodate)  into  the  blood,  after  the 
pressure  had  been  greatly  lowered  by  section  of  the  spinal  cord, 
led  to  its  appearance  in  the  urine ;  and  microscopic  examination 
showed  that  it  had  passed  through  the  epithelial  cells  of  the 
tubules,  not  of  the  glomeruli. 

It  is  found,  however,  that  after  the  removal  of  a  ligature 
applied  to  the  renal  artery  the  urine  is  albuminous,  showing 
plainly  that  the  cells  have  been  injured  by  the  operation  ;  hence 
Heidenhain's  experiment  described  above  is  not  valid  against 
the  blood-pressure  theory.  Moreover,  too  much  must  not  be 
inferred  from  the  action  of  foreign  substances  under  the  ab- 
normal conditions  of  such  an  experiment.  While  some  physi- 
ologists claim  that  the  glomeruli  are  filtering  mechanisms,  they 
explain  that  filtration  is  not  to  be  understood  in  its  ordinary 
laboratory  acceptation,  but  that  the  glomeruli  discriminate  as 
to  what  they  allow  to  pass,  yet  they  in  no  way  explain  how 
this  is  done.  They  make  the  whole  process  depend  on  blood- 
pressure,  and  attribute  little  special  action  to  the  flat  epithelium 
of  the  Malpighian  capsules. 

Though  we  can  not  admit  the  full  force  of  Heidenhain's  ex- 
periments as  he  interprets  them,  we  still  believe  that  his  views 
are  most  in  harmony  with  the  general  laws  of  biology  and  the 


EXCRETION   BY  THE  KIDNEY.  423 

special  facts  of  renal  secretion.  More  recently  it  lias  been  ren- 
dered clear  that  physical  theories  of  the  work  of  the  kidney  can 
not  hold,  even  of  the  glomeruli,  which  are  shown  to  be,  as  we 
should  have  expected,  true  secreting  organs.  Now,  there  can 
be  no  doubt  that  blood-pressure  is  a  most  important  determin- 
ing condition  here  as  in  other  secreting  processes,  in  the  mam- 
mal at  all  events ;  but  whether  of  itself  or  because  of  the  influ- 
ence it  has  on  the  rapidity  of  blood-flow,  it  is  difficult  to  deter- 
mine ;  or  rather  whether  solely  to  the  latter,  for  that  the  con- 
stant supply  of  fresh  blood  is  a  regular  condition  of  normal 
secretion  there  can  be  no  doubt.  Further,  it  seems  probable  that 
blood-pressure  has  more  to  do  with  the  secretion  of  water  than 
any  other  constituent  of  urine.  But  we  maintain  that  it  should 
be  called  a  genuine  secretion,  and  that  nothing  is  gained  by 
using  the  term  "  filtration  " — on  the  contrary,  that  it  is  mislead- 
ing, and  tends  to  divert  attention  from  the  real  though  often 
hidden  nature  of  vital  processes.  The  facts  of  disease  and  the 
evidence  of  therapeutics,  we  think,  all  favor  such  a  view  of  the 
work  of  the  kidneys. 

Nerves  having  an  influence  over  the  secretion  of  urine  simi- 
lar to  those  acting  on  the  digestive  glands  have  not  yet  been 
determined.  The  powerful  influence  of  emotion,  especially 
well  seen  in  the  dog,  over  the  secretion  of  urine  shows  that 
there  must  be  nervous  channels  through  which  the  nerve- 
centers  act  on  the  kidneys ;  though  whether  the  results  are  not 
wholly  dependent  upon  vaso-motor  effects  may  be  considered 
as  an  open  question  by  many.  We  think  such  a  view  im- 
probable in  the  highest  degree.  The  most  recent  investigations 
would  seem  to  show  that  the  vaso-motor  fibers  run  in  the  dor- 
sal nerves,  especially  the  eleventh,  twelfth,  and  thirteenth,  in 
the  dog,  and  that  of  these  the  vaso-constrictors  are  the  best  de- 
veloped. 

Pathological.— When  the  kidneys  are  excised,  the  ureters 
ligatured,  or  when  the  former  are  so  diseased  as  to  be  inca- 
pable of  performing  their  functions,  death  is  the  result,  being 
preceded  by  marked  depression  of  the  brain-centers,  passing 
into  coma.  Exactly  which  of  the  retained  products  brings 
about  these  results  is  not  known.  They  are  likely  due  to  sev- 
eral, and  it  impresses  on  the  mind  the  importance  of  those  pro- 
cesses by  which  the  constantly  accumulating  waste  is  elimi- 
nated. 


424  COMPARATIVE  PHYSIOLOGY. 

THE   EXPULSION   OF   URINE. 

We  now  present  in  concise  form  certain  facts  on  which  to 
base  opinions  as  to  the  nature  of  the  processes  by  which  the 
bladder  is  emptied. 

It  will  be  borne  in  mind  that  the  secretion  of  urine  is  con- 
stant, though  of  course  very  variable,  that  the  urine  is  con- 
veyed in  minute  quantities  by  rhythmically  contractile  tubes 
(ureters)  which  open  into  the  bladder  obliquely ;  and  that  the 
bladder  itself  is  highly  muscular,  the  cells  being  arranged  both 
circularly  and  obliquely,  with  a  special  accumulation  of  the 
circular  fibers  around  the  neck  of  the  organ  to  form  the  sphinc- 
ter vesicae. 

1.  It  is  found  that  the  pressure  which  the  sphincter  of  the 
bladder  can  withstand  in  the  dead  is  much  less  (about  one 
third)  than  in  the  living  subject.  2.  We  are  conscious  of  being 
able  to  empty  the  bladder,  whether  it  contains  much  or  little 
fluid.  3.  We  are  equally  conscious  of  an  urgency  to  evacuation 
of  the  vesical  contents,  according  to  the  fullness  of  the  organ, 
the  quality  of  the  urine,  and  a  variety  of  other  conitions. 

4.  Emotions   may  either  retard  or  render  micturition  urgent. 

5.  In  a  dog  in  which  the  cord  has  been  divided  in  the  dorsal 
region  some  months  previously,  micturition  may  be  induced 
reflexly,  as  by  sponging  the  anus.  6.  In  the  paralyzed  there 
may  be  retention  or  dribbling  of  urine.  7.  In  cases  of  urethral 
obstruction  from  a  calculus,  stricture,  etc.,  there  may  be  excess- 
ive activity  of  the  muscular  tissue  of  the  bladder  walls.  8. 
Evacuation  of  the  bladder  may  occur  in  the  absence  of  con- 
sciousness (sleep). 

The  most  obvious  conclusions  from  these  facts  are  that — 1. 
The  urine  finds  its  way  to  the  bladder  partly  through  muscular 
(peristaltic)  contractions  of  the  ureters,  partly  through  gravity, 
in  man  at  all  events,  and  partly  from  the  pressure  within  the 
tubules  of  the  kidneys  themselves.  2.  The  evacuation  of  urine 
may  take  place  independently  of  the  will  (see  8),  and  is  a  reflex 
(5)  act.  3.  Micturition  may  be  initiated  by  the  will,  which  is 
usually  the  case,  when  by  the  action  of  the  abdominal  muscles 
a  little  urine  is  squeezed  into  the  urethra,  upon  which  afferent 
impulses  set  up  contractions  of  the  bladder  by  acting  on  the 
detrusor  center  of  the  cord  and  at  the  same  time  inhibit  the 
center  presiding  over  the  sphincter  (if  such  there  be),  permit- 
ting of  its  relaxation.     4.  Emotions  seem  to  interfere  with  the 


EXCRETION   BY  THE   KIDNEY. 


425 


Fig.  &0.— Superior  and  general  view  of  the  genito-urinary  apparatus  in  the  stallion 
with  the  axtenesr,(Chauveau).  A,  left  kidney;  B,  right  kiduev  a  b  ureters-  C  C 
suprarenal  capsules;  D,  bladder;  E.  E.  testicles;  e,  head  of  ep'ididimis;  ','  tail 
of  epididimis;  *,  deferent  canal;  G,  pelvic  dilatation  of  deferent  canal-  H  left 
seminal  vesicle;  the  right  has  been  removed,  along  with  the  deferent  canal  of 


426  COMPARATIVE   PHYSIOLOGY. 

same  side,  to  show  insertion  of  ureters  into  bladder;  I,  prostate;  J,  Cowper's 
glands;  K,  membranous  or  intra-pelvic  portion  of  urethral  canal;  L,  its  bulbous 
portion;  M,  cavernous  body  of  penis;  in,  m,  its  roots;  N,  head  of  penis;  1,  ab- 
dominal aorta;  2,2,  arteries  (renal)  giving  off  principal  capsular  artery;  3,  sper- 
matic artery;  4,  common  origin  of  umbilical  and  arteries  of  bulb;  5,  umbilical 
artery;  6,  its  vesical  branch;  7,  internal  artery  of  bulb;  8,  its  vesico-prostatic 
branch. 

ordinary  control  of  the  brain-centers  over  those  in  the  spinal 
cord.  5.  It  may  be  assumed  that  the  normal  tone  of  the 
sphincter  of  the  bladder  is  maintained  reflexly  by  the  spinal 
cord.  The  unwonted  muscular  contraction  associated  with  an 
obstruction  to  the  outflow  of  urine  may  be  in  part  of  nervous 
origin,  but  is  also,  in  all  probability,  owing  in  some  degree  to 
the  muscle-cells  resuming  an  independent  contractility,  due  to 
what  we  recognize  as  the  principle  of  reversion.  The  same  is 
seen  in  the  heart,  ureters,  and  similar  structures. 

Pathological. — There  may  be  incontinence  of  urine  from  pa- 
ralysis, the  cerebral  centers  being  unable  to  control  those  in 
the  spinal  cord.  Dribbling  of  urine  may  be  due  to  retention  in 
the  first  instance,  the  tone  of  the  sphincter  being  finally  over- 
come, owing  to  increase  of  pressure  within  the  bladder.  Over- 
distention  of  the  bladder  may  arise  in  consequence  of  lack  of 
tone  in  the  muscular  walls,  though  this  is  rare.  Strangury  is 
due  to  excessive  action  of  the  walls  of  the  bladder  and  the 
sphincter,  brought  about  reflexly,  when  the  organ  is  unduly 
irritable,  as  in  inflammation,  after  the  abuse  of  certain  drugs 
(cantharides),  etc. 

Comparative. — In  man  the  last  drops  of  urine  are  expelled 
by  the  action  of  the  bulbo-cavernosus  muscle  and  perhaps  some 
others.  In  the  dog  and  many  other  animals  the  regulated  and 
voluntary  use  of  this  muscle,  marked  in  a  high  degree,  produces 
that  interrupted  flow  so  characteristic  of  the  micturition  of 
these  animals. 

Summary. — Urine  is  in  mammals  a  fluid  of  variable  specific 
gravity  and  reaction,  yellow  in  color,  and  containing  certain 
salts,  pigments,  and  nitrogenous  bodies.  The  chief  of  the  latter 
is  urea. 

The  kidneys  and  skin  especially  supplement  one  another, 
and  normally  great  activity  of  the  one  implies  lessened  ac- 
activity  of  the  other.  This  is  availed  of  in  the  treatment  of  dis- 
ease. 

Both  the  Malpighian  capsules  and  the  renal  tubules  have  a 
true  secretory  function,  though  the  larger  pai^t  of  the  water  of 
urine  is  secreted  by  the  former.     Blood-pressure  is  an  important 


EXCRETION  BY  THE   KIDNEY.  427 

condition  of  secretion,  though  it  is  likely  that  this  is  so  chiefly 
because  it  favors  a  rapid  renewal  of  the  blood  circulating 
through  the  organ.  Whether  there  are  nerves  that  influence 
secretion  directly,  as  in  the  case  of  the  skin,  is  not  determined. 
Suppression  of  the  renal  functions  leads  to  symptoms  in 
which  the  nervous  system  is  recognized  as  suffering  to  the 
extent  often  of  coma,  ending  in  death.  The  urine  of  most  other 
animals  is  more  concentrated  than  that  of  man  ;  this  secretion 
in  carnivora  being  acid,  and  in  herbivora  alkaline  in  reaction. 


THE   METABOLISM   OF   THE  BODY. 


In  the  widest  sense  the  term  metabolism  may  he  conven- 
iently applied  to  all  the  numerous  changes  of  a  chemical  kind, 
resulting  from  the  activity  of  the  protoplasm  of  any  tissue  or 
oi*gan.  In  a  more  restricted  meaning  it  is  confined  to  changes 
undergone  hy  the  food  from  the  time  it  enters  till  it  leaves  the 
body,  in  so  far  as  these  are  not  the  result  of  obvious  mechanical 
causes.  The  sense  in  which  it  is  employed  in  the  present 
chapter  will  be  plain  from  the  context,  though  usually  we  shall 
be  concerned  with  those  changes  effected  in  the  as  yet  compara- 
tively unprepared  products  of  digestion,  by  which  they  are  ele- 
vated to  a  higher  rank  and  brought  some  steps  nearer  to  the 
final  goal  toward  which  they  have  been  tending  from  the  first. 
As  yet  our  attempts  to  trace  out  these  steps  have  been  little 
better  than  the  fruitless  efforts  of  a  lost  traveler  to  find  a  road, 
the  general  direction  of  which  he  knows,  but  the  ways  by  which 
it  is  reached  only  the  subject  of  plausible  conjecture.  We 
shall  therefore  not  discuss  the  subject  at  length  from  this  point 
of  view. 

THE   METABOLISM   OF   THE   LIVER. 

This  organ  has  two  well-recognized  functions  :  1.  The  for- 
mation of  bile.  2.  The  formation  of  glycogen.  We  have 
already  considered  the  first. 

Glycogen  may  be  obtained  from  the  liver  of  mammals  as  a 
whitish  amorphous  powder,  having  the  chemical  composition  of 
starch,  and  has  in  fact  been  termed  animal  starch. 

By  appropriate  treatment  it  may  be  converted  into  sugar  by 
a  process  of  hydration  (CoHioOc  +  H2O  =  CoHiaOo). 

The  principal  facts  as  to  the  storage  of  glycogen  in  the  liver 
may  be  briefly  stated  thus  : 

1.  Glycogen  has  been  found  in  the  liver  of  a  large  number 


THE  METABOLISM   OF   THE   BODY.  429 

of  groups  of  animals  including  some  invertebrates.  2.  Among 
mammals  it  is  most  abundant  wben  the  animal  feeds  largely 
on  carbohydrates.  3.  It  is  found  in  the  liver  of  the  carnivora, 
and  in  those  of  omnivora,  when  feeding  exclusively  on  flesh. 
4.  When  an  animal  starves  (does  not  feed),  the  glycogen  grad- 
ually disappears.  5.  A  fat-diet  does  not  give  rise  to  glycogen. 
6.  During  early  foetal  life  glycogen  is  found  in  all  the  tissues, 
but  later  it  is  restricted  more  and  more  to  the  liver,  though 
even  in  adixlts  it  is  to  be  found  in  various  tissues,  especially  the 
muscles,  from  which  it  is  almost  never  absent. 

From  the  facts  the  inference  is  plain  that  glycogen  is  formed 
from  carbohydrate  materials  ;  or,  to  be  rather  more  cautious, 
that  the  formation  of  this  substance  is  dependent  on  the  pres- 
ence of  such  material  in  the  food. 

The  Uses  of  Glycogen. — No  positive  statement  can  be  made  on 
this  subject.   It  is  generally  believed  to  be  transformed  into  sugar. 

What  is  the  fate  of  the  transformed  glycogen  ?  What  be- 
comes of  the  sugar  ?  We  can  answer,  negatively,  that  it  is  not 
used  up  in  the  blood,  it  is  not  oxidized  there  ;  but  by  what 
tissues  it  is  used  or  how  it  is  made  available  in  the  economy  is 
a  subject  on  which  we  are  profoundly  ignorant.  The  presence 
of  so  much  glycogen  in  the  partially  developed  tissues  of  the 
foetus  points  to  its  importance,  and  suggests  its  being  a  crude 
material  which  is  laid  up  to  be  further  elaborated,  as  in  vege- 
tables, by  the  growing  protoplasm. 

METABOLISM   OF   THE    SPLEEN. 

The  physiological  significance  of  the  peculiar  structure  of 
this  organ,  though  not  yet  fully  understood,  is  much  plainer 
than  it  was  till  recently.  The  student  is  recommended  to  look 
carefully  into  the  histology  of  the  spleen,  especially  the  dis- 
tribution of  its  muscular  tissue  and  the  peculiarities  of  its 
blood-vascular  system.  It  has  already  been  pointed  out  that 
there  is  little  doubt  that  leucocytes  are  manufactured  here  even 
in  the  adult,  possibly  also  red  cells ;  and  that  the  latter  are  dis- 
integrated, and  the  resulting  substances  worked  over,  possibly 
by  this  organ  itself.  This  view  is  rendered  probable,  not  only 
by  microscopic  study  of  the  organ,  but  by  a  chemical  examina- 
tion of  the  splenic  pulp;  for  a  ferruginous  proteid,  and  numer- 
ous pigments,  of  a  character  such  as  harmonizes  with  this  con- 
ception, are  found, 


430 


COMPARATIVE   PHYSIOLOGY. 


The  fact  that  the  spleen-pulp  does  not  agree  in  composition 
with  either  blood  or  serum ;  that  it  abounds  in  extractives  such 


Fig.  321. — Vertical  section  of  a  small  superficial  portion  of  human  spleen,  seen  with 
low  power  (Schafer).  A,  peritoneal  and  fibrous  covering;  b,  trabecules;  c,  c,  Mal- 
pighian  corpuscles,  in  one  of  which  an  artery  is  seen  cut  transversely,  in  the  other, 
longitudinally;  d,  injected  arterial  twigs;  e,  spleen-pulp. 

as  lactic,  butyric,  formic,  and  acetic  acids,  together  with  inosit, 
xanthin,  hypoxanthin,  leuciu  and  uric  acid — points  to  its  being 


Pio.  322.— Thin  section  of  spleen-palp,  highly  magnified,  showing  mode  of  origin  of 
a  small  vein  in  the  interstices  of  pulp  (Scnafer).  v,  vein  filled  with  blood-corpus- 
cles, which  are  in  continuity  with  others,  hi.  filling  up  interstices  of  retiform  tissue 
of  pulp;  w,  wull  of  vein.  The  shaded  bodies  among  red  corpuscles  are  pale  cor- 
puscles. 


THE   METABOLISM   OF   THE   BODY. 


431 


the  seat  of  a  complex  metabolism,  though  neither  the  changes 
themselves  nor  their  purpose  are  well  understood. 

Nevertheless,  it  must  be  admitted  that  to  recognize  this  was 
a  great  advance  upon  the  view  that  the  spleen  had  no  impor- 
tant function,  and  that  this  was  shown  by  the  removal  of 
the  organ  without 
change  in  the  ani- 
mal's economy. 

But  to  believe 
that  there  are  no 
such  changes,  and  to 
have  clear  proof  of 
it,  are  two  different 
things.  As  a  matter 
of  fact,  closer  study 
does  show  that  in 
some  animals  there 
are  alterations  in  the 
lymphatic  glands 
and  bone  -  marrow, 
which  organs  are 
undoubtedly  manu- 
facturers of  blood- 
cells. 

These       changes 


Fig.  323. — Portion  of  spleen  of  cat,  showing  Malpighian 
(lymphatic)  corpuscle  (after  Cadiat).  A,  artery 
around  which  corpuscle  is  placed;  B,  meshes  of 
spleen-pnip,  injected;  C,  artery  of  corpuscle  ramify- 
ing in  lymphatic  tissue.  The  clear  space  around 
corpuscle  represents  a  lymphatic  sinus. 


are  unquestionably  compensatory,  and  that  other  similar  ones 
corresponding  to  comparatively  unknown  functions  of  the 
spleen  have  not  as  yet  been  discovered  is  owing  likely  to  our 
failures  rather  than  their  real  absence.  We  dwell  for  a  mo- 
ment on  this,  because  it  illustrates  the  danger  of  the  sort  of  rea- 
soning that  has  been  applied  in  the  case  of  this  and  other  or- 
gans; and  it  shows  the  importance  of  recognizing  the  force 
of  the  general  pi'inciples  of  biology,  and  also  the  desirability 
of  refraining  from  drawing  conclusions  that  are  too  wide 
for  the  premises.  In  every  department  of  physiology  it  must 
be  more  and  more  recognized  that  what  is  true  of  one  group 
of  animals  is  not  necessarily  true  of  another,  or  even  of  other 
individuals,  though  the  differences  in  the  latter  case  are  of 
course  usually  less  marked.  We  have  referred  to  this  be- 
fore, and  shall  do  so  again,  for  it  is  as  yet  but  too  little  con- 
sidered. 


432  COMPARATIVE  PHYSIOLOGY. 

THE   CONSTRUCTION    OF   FAT. 

It  is  a  well-known  fact  that,  speaking  generally,  a  diet  rich 
in  carbohydrates  favors  fat  formation,  hoth  in  man  and  other 
animals;  though  it  is  not  to  be  forgotten  that  many  persons 
seem  to  be  unable  to  digest  such  food,  or,  at  all  events,  to  as- 
similate it  so  as  to  form  fat  to  any  great  extent.  Persons  given 
to  excessive  fat  production  are  as  frequently  as  not  sparing 
users  of  fat  itself. 

It  is  possible  in  man  and  probable  in  ruminants  that  fer- 
mentations may  occur  in  the  intestines  giving  rise  to  fatty  acids 
which  are  possibly  converted  into  fats  by  the  cells  of  the  villi 
or  elsewhere.  Certain  feeding  experiments  favor  the  view 
that  carbobydrates  may  be  converted  into  fat  or  in  some  way 
give  rise  to  an  increase  in  this  substance  ;  for  it  is  to  be  borne 
in  mind  that  fat  may  arise  from  a  certain  diet  in  various 
ways  other  than  its  direct  transformation  into  this  substance 
itself. 

There  are  certain  facts  that  make  it  clear  that  fat  can  be 
formed  from  proteids:  1.  A  cow  will  produce  more  butter  than 
can  be  accounted  for  by  the  fat  in  her  food  alone.  2„  A  bitch 
which  had  been  fed  on  meat  produced  more  fat  in  her  milk 
than  could  have  been  derived  directly  from  her  food,  and  this, 
when  the  animal  was  gaining  in  weight,  which  is  usually  to  be 
traced  to  the  addition  of  fat ;  so  that  the  fat  of  the  milk  was 
not,  in  all  probability,  derived  from  that  of  the  dog's  body ; 
and,  as  will  be  seen  presently,  can  be  accounted  for  without 
such  a  supposition.  3.  It  has  been  shown  by  analysis  that  472 
parts  of  fat  were  deposited  in  the  body  of  a  pig  for  every  100  in 
its  food. 

These  facts  of  themselves  suffice  to  show  that  fat  can  be 
formed  from  proteid,  or  at  least  that  proteid  food  can  of  itself 
give  rise  to  a  metabolism,  resulting  in  fat  formation;  and  the 
latter  is  probably  the  better  way  to  state  the  case  in  the  present 
condition  of  knowledge. 

That  fat  is  a  real  formation,  dependent  for  its  composition 
on  the  work  of  living  tissues,  is  clear  from  the  well-known  fact 
that  the  fat  of  one  animal  differs  from  that  of  another,  and  that 
it  preserves  its  identity,  no  matter  what  the  food  may  be,  or.  in 
what  form  fat  itself  may  be  provided.  Certain  constituents  of 
the  animal's  fat  may  be  wholly  absent  from  the  fat  of  its  food, 
yet  they  appear  just  the  same  in  the  fat  produced  under  such 


THE   METABOLISM   OF   THE   BODY.  433 

diet.     Even  bees  can  construct  their  wax  from  proteicl,  or  use 
unlike  substances,  as  sealing-wax. 


Pig.  324. — Section  of  mammary  gland  (udder  and  nipplel  of  cow  (after  Thanhoffcr)- 
Ma,  substance  of  gland;  N  nipple;  A,  acini  of  gland;  m.  d,  milk-ducts;  C,  milk- 
cisterns;/,  folds  in  wide  milk-ducts;  S,  section  of  sphincter  muscle;  s,  external 
skin;  n.  m.  d,  narrow  milk-duct  in  nipple. 

But  histological  examination  of  forming  adipose  tissue  itself 
throws  much  light  upon  the  subject.  Fat-cells  are  those  in 
which  the  protoplasm  has  been  largely  replaced  by  fat.     The 

28 


434 


COMPARATIVE   PHYSIOLOG-Y. 


latter  is  seen  to  arise  in  the  former  as  very  small  globules 
which  run  together  more  and  more  till  they  may  wholly  re- 
place the  original  protoplasm. 

The  history  of  the  mammary  gland  is,  perhaps,  still  more 
instructive.  In  this  case,  the  appearance  of  the  cells  during 
lactation  and  at  other  periods  is  entirely  different.  Fat  may  he 
seen  to  arise  within  these  cells  and  be  extruded,  perhaps  in  the 
same  way  as  an  Amoeba  gets  rid  of  the  waste  of  its  food.  So 
far  as  the  animal  is  concerned,  milk  is  an  excretion  in  a  limited 
sense. 

It  is,  in  the  nature  of  the  case,  impossible  to  follow  with 
the  eye  the  formation  and  separation  of  milk-sugar,  casein,  etc. 


Fig.  325. — I.  Acinus  from  mamma  of  a  bitch  when  inactive  (after  Heidenhain).  II. 
During  secretion  of  milk,  a,  b,  milk-globules;  c,  d,  e,  colostrum-corpuscles;  /, 
pale  cells. 


But  the  whole  process  is  plainly  the  work  of  the  cells,  and  in 
no  mechanical  sense  a  mere  deposition  of  fat,  etc.,  from  the 
blood  ;  and  the  same  view  applies  to  the  construction  of  fat  by 
connective  (adipose)  tissue. 

Whether  fat,  as  such,  or  fatty  acid,  is  dealt  with  without 
being  built  up  into  the  protoplasm  of  the  cell,  is  not  known  ; 
but,  taking  all  the  facts  into  the  account,  and  considering  the 
behavior  of  cells  generally,  it  seems  most  natural  to  regard  the 
construction  of  fat  as  a  sort  of  secretion  or  excretion.  To  sup- 
pose that  a  living  cell  acts  upon  material  in  the  blood  as  a 
workman  in  a  factory  on  his  raw  material,  or  even  as  a  chemist 
does  in  the  laboratory,  seems  to  be  too  crude  a  conception  of 
vital  processes.  Until  it  can  be  rendered  very  much  clearer 
than  at  present,  it  is  not  safe  to  assume  that  their  chemistry  is 
our  chemistry,  or  their  methods  our  methods.  It  maybe  so; 
but  let  us  not,  in  our  desire  for  simple  explanations  or  undue 
haste  to  get  some  sort  of  theory  that  apparently  fits  into  our 


THE   METABOLISM   OF   THE   BODY. 


435 


own  knowledge,  assume  it  gratuitously,  in  the  absence  of  the 
clearest  proofs,  especially  when  our  failures  on  this  supposition 
are  so  numerous. 

We  may  say,  then,  that  fat  is  not  merely  selected  from  the 
blood,'  but  formed  in  the  animal  tissues  ;    that  fat  formation 


Fig.  326  —Microscopic  appearances  of— I.  milk;  II,  cream;  III,  butter;  IV,  colostrum 
of  mare;  V,  colostrum  of  cow  (after  Thanhoffer). 

may  take  place  when  the  food  consists  largely  of  carbohydrates, 
when  it  is  chiefly  proteid,  or  when  proteid  and  fatty.  In  other 
words,  fat  results  from  the  metabolism  of  certain  cells,  which 
is  facilitated  by  the  consumption  of  carbohydrate  and  fatty 
food,  but  is  possible  when  the  food  is  chiefly  nitrogenous.  We 
must,  however,  recognize  differences  both  of  the  species  and  the 
individual  in  this  respect,  as  to  the  extent  to  which  one  kind  of 
food  or  the  otber  most  favors  fat  formation  (excretion).  The 
use  of  the  adipose  tissue  as  a  packing  to  prevent  undue  escape 
of  heat  is  evident  ;  but  more  important  purposes  are  probably 
served,  as  will  appear  from  later  considerations. 

Pathological.— Excessive  fat  formation,  leading  to  the  ham- 
pering of  respiration,  the  action  of  the  muscles,  and,  to  a  certain 
extent,  many  other  functions  of  the  body,  does  not  arise  in  man 


436  COMPARATIVE   PHYSIOLOGY. 

usually  till  after  middle  life,  when  the  organism  has  seen  its 
best  days.  It  seems  to  indicate,  if  we  judge  by  the  frequency 
of  fatty  degeneration  after  disease,  that  the  protoplasm  stops 
short  of  its  best  metabolism,  and  becomes  degraded  to  a  lower 
rank  ;  for  certainly  adipose  tissue  does  not  occupy  a  high 
place  in  the  histological  scale.  Such  pathological  facts  throw 
a  good  deal  of  light  upon  the  general  nature  of  fat  excre- 
tion, as  it  would  be  better  to  term  it,  perhaps,  and  seem  to 
warrant  the  view  that  we  have  presented  of  the  metabolic  pro- 
cesses. 

Although  the  nerves  governing  the  secretion  of  milk  have 
not  been  traced,  there  can  be  no  doubt  that  the  nervous  system 
controls  this  gland  also.  The  influence  of  the  emotions  on  both 
the  quantity  and  quality  of  the  milk  in  the  human  subject  and 
in  lower  animals  is  well  known. 

Comparative. — While  breeders  recognize  certain  foods  as 
tending  to  fat  formation  and  others  to  milk  production,  it  is 
interesting  to  note  that  their  experience  shows  that  race  and 
individuality,  even  on  the  male  side,  tell.  The  same  conditions 
being  in  all  respects  observed,  one  breed  of  cows  gives  more 
and  better  milk  than  another,  and  the  bull  is  himself  able  to 
transmit  this  peculiarity ;  for,  when  crossed  with  inferior  breeds, 
he  improves  the  milking  qualities  of  the  latter.  Individual 
differences  are  also  well  known. 


THE    STUDY   OF    THE    METABOLIC   PROCESSES  BY 
OTHER   METHODS. 

It  will  be  abundantly  evident  that  our  attempts  to  follow 
the  changes  which  the  food  undergoes  from  the  time  of  its 
introduction  into  the  blood  until  it  is  removed  in  altered  form 
from  the  body  has  not  been  as  yet  attended  with  great  success. 
It  is  possible  to  establish  relations  between  the  ingesta  and  the 
egesta,  or  the  income  and  output  which  have  a  certain  value. 
It  is  important,  however,  to  remember  that,  when  quantitive 
estimations  have  to  be  made,  a  small  error  in  the  data  becomes 
a  large  error  in  the  final  estimate  ;  one  untrue  assumption 
may  vitiate  completely  all  the  conclusions. 

In  discussing  the  subject  we  shall  introduce  a  number  of 
tables,  but  it  will  be  remembered  that  the  results  obtained  by 
one  investigator  differ  from  those  obtained  by  another  ;  and 
tbat  in  all  of  tbem  there  are  some  deviations   from  strict  ac- 


THE  METABOLISM   OP   THE   BODY.  437 

curacy,  so  that  the  results  must  he  regarded  as  only  approxi- 
mately correct.  It  is,  however,  we  think,  better  to  examine 
such  statistical  tables  of  analyses,  etc.,  than  to  rely  on  the  mere 
verbal  statement  of  certain  results,  as  it  leaves  more  room  for 
individual  judgment  and  the  assimilation  of  such  ideas  as  they 
may  suggest  outside  of  the  subject  in  hand. 

The  subject  of  diet  is  a  very  large  one  ;  but  it  will  be  evi- 
dent on  reflection  that,  before  an  average  diet  can  be  prescribed 
on  any  scientific  grounds,  the  composition  of  the  body  and  the 
nature  of  those  processes  on  which  nutrition  generally  depends 
must  be  known.  .  Not  a  little  may  be  learned  by  an  examina- 
tion of  the  behavior  of  the  body  in  the  absence  of  all  diet, 
when  it  may  be  said  to  feed  on  itself,  one  tissue  supplying 
another.  All  starving  animals  are  in  the  nature  of  the  case 
carnivorous. 

For  the  cat  an  analysis  has  yielded  the  following  : 

Muscle  and  tendons. 45  "0  per  cent. 

Bones 147 

Skin 12-0 

Mesentery  and  adipose  tissue 3*8 

Liver 4*8 

Blood  (escaping  at  death) 6'0 

Other  organs  and  tissues 13'7 

The  large  proportional  weight  of  the  muscles,  the  similarly 
large  amount  of  blood  they  receive,  which  is  striking  in  the 
case*  of  the  liver,  also  suggest  that  the  metabolism  of  these 
structures  is  very  active,  and  we  should  expect  that  they 
would  lose  greatly  during  a  starvation  period.  It  is  a  matter  of 
common  observation  that  animals  do  lose  weight  and  grow 
thin  under  such  circumstances,  which  means  that  they  must 
lose  in  the  muscles  and  the  adipose  tissue.  Attempts  have  been 
made  to  determine  exactly  the  extent  to  which  the  various 
tissues  do  suffer  during  complete  abstinence  from  food,  and 
this  may  be  gathered  from  the  table  given  below. 

It  will  not  be  forgotten  that  about  three  fourths  of  the  body 
is  made  up  of  water,  so  that  the  loss  of  a  lai'ge  amount  of  the 
latter  during  starvation  is  to  be  expected. 

In  the  case  of  a  cat  during  a  starvation  period  of  thirteen 
days  734  grammes  of  solids  were  lost,  of  which  248  grammes 
were  fat  and  118  muscle — i.  e.,  about  one  half  of  the  total  loss 
was  referable  to  these  two  tissues  alone. 

The  other  tissues  lost  as  follows,  estimated  as  dry  solids  : 


438  COMPARATIVE   PHYSIOLOGY. 

Adipose  tissues 97-0  per  cent. 

Spleen 63-l        " 

Liver 56-6 

Muscles 30-2 

Blood 17-6 

Brain  and  spinal  cord 0-0        " 

It  will  be  observed  (a)  that  the  loss  of  the  fatty  tissue  was 
greatest,  nearly  all  disappearing ;  (b)  that  the  grandular  struct- 
ures were  next  in  order  the  greatest  sufferers  ;  (c)  that  after 
them  come  the  skeletal  muscles. 

Now,  it  has  been  already  seen  that  these  tissues  all  engage 
in  an  active  metabolism  with  the  exception  of  adipose  tissue. 

The  small  loss  on  the  part  of  the  heart,  which  is  still  less  for 
the  nervous  system,  is  especially  noteworthy.  The  loss  of  adi- 
pose tissue  is  so  striking  that  we  must  regard  it  as  an  especially 
valuble  storehouse  of  energy,  available  as  required. 

When  we  turn  to  the  urine  for  information,  it  is  found  that 
in  the  above  case  27  grammes  of  nitrogen  were  excreted  and 
almost  entirely,  of  course,  in  the  form  of  urea;  and  since  the 
loss  of  nitrogen  from  the  muscles  amounted  to  15  grammes,  it 
will  appear  that  more  than  one  half  of  the  nitrogenous  excreta 
is  traceable  to  the  metabolism  of  muscular  tissue.  It  has  been 
customary  to  account  for  the  urea  in  two  ways :  first,  as  derived 
from  the  metabolism  of  the  tissues  as  stich,  and  continuously 
throughout  the  whole  starvation  period ;  and,  secondly,  from  a 
stored  surplus  of  proteid  which  was  assumed  to  be  used  up 
rapidly  during  the  early  clays  of  the  fasting,  and  was  the  luxus 
consumption  of  certain  investigators. 

Comparative.  —  Experiment  has  shown  that  the  length  of 
time  during  which  different  groups  of  animals  can  endure  com- 
plete withdrawal  of  food  is  very  variable,  and  this  applies  to 
individuals  as  well  as  species.  That  such  differences  hold  for 
the  human  subject  is  well  illustrated  by  the  history  of  the  sur- 
vivors of  wrecks.  Making  great  allowances  for  such  devia- 
tions from  any  such  results  as  can  be  established  by  a  limited 
number  of  experiments,  it  may  be  stated  that  the  human  being 
succumbs  in  from  twenty-one  to  twenty-four  days  ;  dogs  in 
good  condition  at  the  outset  in  from  twenty-eight  to  thirty 
days;  small  mammals  and  birds  in  nine  days,  and  frogs  in 
nine  months.  Very  much  depends  on  whether  water  is  allowed 
or  not — life  lasting  much  longer  in  the  former  case.  The  very 
young  and  the  very  old  yield  sooner  than  persons  of  middle 


THE  METABOLISM  OF  THE   BODY.  439 

age.  It  has  been  estimated  that  strong  adults  die  when  they 
lose  t4q  of  the  body-weight.  Well-fed  animals  lose  weight  more 
rapidly  at  first  than  afterward. 

Diet. — All  experiments  and  observations  tend  to  show  that 
an  animal  can  not  remain  in  health  for  any  considerable  period 
without  having  in  its  food  proteids,  fats,  carbohydrates,  and 
salts;  indeed,  sooner  or  later  deprivation  of  any  one  of  these 
will  result  in  death. 

Estimates  based  on  many  observations  have  been  made  of 
the  proportion  in  which  these  substances  should  enter  into  a 
normal  diet. 

For  the  herbivora  from  1  to  8-9  (some  claim  1  to  5|)  is  the 
estimated  ratio  of  nitrogenous  to  non-nitrogenous  foods ;  and  2 
of  the  former  to  1  of  fat. 

One  conclusion  that  is  obvious  from  analysis  of  foods  is  that, 
in  order  to  obtain  the  amount  of  proteids  needed  from  certain 
kinds,  enormous  quantities  must  be  eaten  and  digested ;  and  as 
there  would  be  in  such  cases  an  excess  of  carbohydrates,  fats, 
etc.,  unnecessary  work  is  entailed  upon  the  organism  in  order 
to  dispose  of  this ;  so  that  to  feed  a  working  horse  entirely  on 
grass,  a  dog  wholly  on  porridge,  or  a  man  on  bread  would  be 
very  unwise. 

FEEDING   EXPERIMENTS   (Ingesta  and  Egesta). 

If  all  that  enters  the  body  in  any  form  be  known,  and  all 
that  leaves  it  be  equally  well  known,  conclusions  may  be  drawn 
in  regard  to  the  metabolism  the  food  has  undergone.  The  pos- 
sible sources  of  fallacy  will  appear  as  we  proceed. 

The  ingesta,  in  the  widest  sense,  include  the  respired  air  as 
well  as  the  food  ;  though  from  the  latter  must  be  subtracted 
the  waste  (undigested)  matters  that  appear  in  the  fasces.  The 
ingesta  when  analyzed  include  carbon,  hydrogen,  oxygen,  ni- 
trogen, sulphur,  phosphorus,  water,  and  salts,  their  source  being 
the  atmosphere  and  the  food-stuffs. 

The  egesta,  the  same,  and  chiefly  in  the  form  of  carbonic  an- 
hydride, of  water  from  the  lungs,  skin,  alimentary  canal,  and 
kidneys,  of  salts  and  water  from  the  skin  and  kidneys,  and  of 
nitrogen,  chiefly  as  urea  almost  wholly  from  the  kidneys.  Usu- 
ally in  experimental  determinations  the  total  quantity  of  the 
nitrogen  of  the  urine  is  estimated.  If  free  nitrogen  plays  any 
part  in  the  metabolic  processes  it  is  unknown. 


440  COMPARATIVE   PHYSIOLOGY. 

A  large  number  of  feeding  experiments  have  been  made  by 
different  investigators,  chiefly,  though. not  exclusively,  on  the 
lower  animals.  Some  such  method  as  the  following  has  usu- 
ally been  pursued:  1.  The  food  used  is  carefully  weighed  and  a 
sample  of  it  analyzed,  so  that  more  exact  data  may  be  obtained. 
2.  The  amount  of  oxygen  used  and  carbonic  anhydride  exhaled, 
as  well  as  the  amount  of  water  given  off  in  any  form  is  esti- 
mated. 3.  The  amount  of  the  nitrogenous  excreta  is  calculated, 
chiefly  from  an  analysis  of  the  urine,  though  any  loss  by  hair, 
etc.,  is  also  to  be  taken  into  account. 

It  has  been  generally  assumed  that  the  nitrogen  of  the  ex- 
creta represents  practically  the  whole  of  that  element  entering 
the  body.     This  has  been  denied  by  some  investigators. 

The  respiratory  products  have  been  estimated  in  various 
ways.  One  consists  in  measuring  the  quantity  of  oxygen  sup- 
plied to  the  chamber  in  which  the  animal  under  observation  is 
inclosed,  and  analyzing  from  time  to  time  samples  of  the  air  as 
it  is  drawn  through  the  chamber;  and  on  these  results  the  total 
estimates  are  based. 

It  will  appear  that  even  errors  in  calculating  the  composi- 
tion of  the  food — and  this  is  very  variable  in  different  samples, 
e.  g.,  of  flesh;  or  any  errors  in  the  analysis  of  the  urine,  or  in 
the  more  difficult  task  of  estimating  the  respiratory  products, 
may,  when  multiplying  to  get  the  totals,  amount  to  serious  de- 
partures from  accuracy  in  the  end ;  so  that  all  conclusions  in 
such  a  complicated  case  must  be  drawn  with  the  greatest  cau 
lion.  But  it  can  not  be  doubted  that  such  investigations  have 
proved  of  much  practical  and  some  scientific  value.  The  labor 
they  entail  is  enormous. 

Nitrogenous  Equilibrium.— It  is  possible  to  so  feed  an  ani- 
mal, say  a  dog,  that  the  total  nitrogen  of  the  ingesta  and  egesta 
shall  be  equal;  and  this  may  be  accomplished  without  the  ani- 
mal losing  or  gaining  weight  appreciably  or  again  while  he  is 
gaining.  If  there  be  a  gain,  it  can  usually  be  traced  to  the 
formation  of  fat,  so  that  the  proteid,  we  may  suppose,  has  been 
split  up  into  a  part  that  is  constructed  into  fat  and  a  part  which 
is  represented  by  the  urea,  the  fat  being  either  used  up  or  stored 
in  the  body.  Moreover,  an  analysis  of  a  pig  that  had  been  fed 
on  a  fixed  diet,  and  a  comparison  made  with  one  of  the  same 
litter  killed  at  the  commencement  of  the  experiment,  showed 
that  of  the  dry  nitrogenous  food  only  about  seven  per  cent  in 
this  animal,  and  four  per  cent  in  the  sheep  had  been  laid  away 


THE   METABOLISM   OF   THE    BODY.  441 

as  dry  proteid.     It  is  perfectly  plain,  then,  that  proteid  diet 
does  not  involve  only  proteid  construction  within  the  body. 

Comparative. — The  amount  of  flesh  which  a  dog,  being  a 
carnivorous  animal,  can  digest  and  use  for  the  maintenance  of 
his  metabolic  processes  is  enormous;  though  it  lias  been  learned 
that  ill-nourished  dogs  can  not  even  at  the  outset  of  a  feeding 
experiment  of  this  kind  maintain  the  equilibrium  of  their  body 
weight  on  a  purely  flesh  diet  (fat  being  excluded).  They  at 
once  commence  to  lose  weight — i.  e.,  they  draw  upon  their  own 
limited  store  of  fat. 

The  digestion  of  herbivora  being  essentially  adapted  to  a 
vegetable  diet,  they  can  not  live  at  all  upon  flesh,  while  a  dog 
can  consume  for  a  time  without  manifest  harm  ?15  to  ^  of  its 
body-weight  of  this  food. 

Man,  when  fed  exclusively  on  meat  soon  shows  failure,  he 
being  unable  to  digest  enough  to  supply  the  needed  cai'bohy- 
drates,  etc.  But  the  large  amount  of  urea  in  the  urine  of  car- 
nivorous animals  generally,  and  the  excess  found  in  the  urine 
of  man  when  feeding  largely  on  a  flesh  diet,  show  that  the  pro- 
teid metabolism  is  under  such  circumstances  very  active. 

It  is  also  a  well-known  observation  that  carnivorous  ani- 
mals (dogs)  are  more  active  and  display  to  a  greater  extent 
their  latent  ferocity,  evidence  of  their  descent  from  wild  car- 
nivorous progenitors,  when  like  them  they  feed  very  largely  on 
flesh.  The  evidence  seems  to  point  pretty  clearly  to  the  con- 
clusion that  a  nitrogenous  (flesh)  diet  increases  the  activity  of 
the  vital  processs  of  the  body,  and  especially  the  proteid  me- 
tabolism. 

But  in  all  these  considerations  it  must  be  borne  in  mind  that 
the  metabolic  processes  go  on  in  the  tissues  and  not  in  the 
blood,  and  probably  not  in  the  lymph.  Not  that  these  fluids 
(tissues)  are  without  their  own  metabolic  processes  for  and  by 
themselves;  but  what  is  meant  to  be  conveyed  is  that  the  met- 
abolic processes  of  the  body  generally  do  not  take  place  in  the 
blood. 

The  Effects  of  Gelatin  in  the  Diet.— Actual  experiment  shows 
that  this  substance  can  not  take  the  place  of  proteid,  though  it 
also  makes  it  evident  that  less  of  the  latter  suffices  when  mixed 
with  a  certain  proportion  of  gelatin.  It  will  be  borne  in  mind 
that  ordinary  flesh  contains,  as  we  find  it  naturally  in  the  car- 
cass, not  only  some  fat,  but  a  good  deal  of  fibrous  tissue,  which 
can  be  converted  by  heating  into  gelatin. 


442  COMPARATIVE  PHYSIOLOGY. 

Fats  and  Carbohydrates.— It  is  a  matter  of  common  observa- 
tion and  of  more  exact  experiment  tbat  even  a  carnivorous  ani- 
mal thrives  better  on  a  diet  of  fat  and  lean  meat  than  on  lean 
flesh  alone.  Thus,  it  has  been  found  that  nitrogenous  equi- 
librium was  as  readily  established  by  a  due  mixture  of  fat  and 
lean  as  upon  twice  the  quantity  of  lean  flesh  alone.  It  is  plain, 
then,  that  the  metabolism  is  actually  slowed  by  a  fatty  diet. 
When  an  animal  is  given  but  little  fat,  none  whatever  is  laid 
up,  but  all  the  carbon  of  the  fat  can  be  accounted  for  in  the 
excreta,  chiefly  as  carbonic  anhydride.  Again,  the  fatty  por- 
tion remaining  constant,  it  has  been  found  that  increasing  the 
proteid  leads  not  to  a  storage  of  the  carbon  of  the  proteid  ex- 
cess, but  to  an  increased  consumption  of  this  element.  It  is 
then  possible  to  understand  how  excessive  consumption  of  pro- 
teids  may  lead,  as  seems  to  be  the  case,  to  the  disappearance  of 
fat  and  loss  of  weight,  so  that  a  proteid  diet  increases  not  only 
nitrogenous  but  non-nitrogenous  metabolism.  That  carbohy- 
drates mixed  with  a  due  proportion  of  the  other  constituents 
of  a  diet  do  increase  fat  formation  is  well  established ;  though 
there  is  no  equally  well-grounded  explanation  of  how  this  is 
accomplished.  Upon  the  whole,  it  seems  most  likely  that  fat 
can  be  directly  formed  from  carbohydrates,  or,  at  all  events, 
that  they  directly  give  rise  to  fat  if  they  are  not  converted 
themselves  into  that  substance. 

Comparative, — It  is  found  that  there  are  relations  between 
the  food  used  and  the  quantity  of  carbonic  dioxide  expelled 
which  are  instructive.  The  formula  following  show  the  amount 
of  oxygen  necessary  to  convert  a  starch  and  a  fat  into  carbonic 
anhydride  and  water : 

1.  CoH1005  +  012=6(C02)+5(H20). 

2.  CeiH.cOe  +  O10u= 57(C02)  +  52(H20). 

It  will  be  observed  that  in  the  first  case  the  oxygen  used  to 
oxidize  the  starch  has  all  reappeared  as  C02,  while  in  the  sec- 
ond only  114  parts  out  of  160  so  reappear.  As  a  matter  of  fact, 
more  of  the  oxygen  used  does  in  herbivora  reappear  as  C02, 
and  less  as  water,  while  the  reverse  holds  for  the  carnivora,  the 
proportion  being,  it  is  estimated,  as  ninety  to  sixty  per  cent. 
This  is  to  be  explained  by  the  character  of  the  food  in  each 
instance,  for  this  relation  no  longer  holds  during  fasting,  when 
the  herbivorous  animal  becomes  carnivorous  in  the  sense  that 
it  consumes  its  own  tissues. 


THE   METABOLISM   OF  THE   BODY.  443 

The  Effects  of  Salts,  Water,  etc.,  in  the  Diet.— When  we 
come  to  inquire  as  to  the  part  salts  play  when  introduced  into 
the  blood,  we  soon  find  that  our  knowledge  is  very  limited. 

Sulphur,  and  especially  phosphorus,  seem  to  have  some  im- 
portant use  which  quite  eludes  detection.  It  is  important  to 
remember  that  certain  salts  are  combined  with  proteids  in  the 
body,  possibly  to  a  greater  extent  than  we  can  learn  from  the 
mere  analysis  of  dead  tissues. 

Pathological. — The  withdrawal  of  any  of  the  important  salts 
of  the  body  soon  leads  to  disease,  clear  evidence  in  itself  of  their 
great  importance.  This  is  notably  the  case  in  scurvy,  in  which 
disease  the  blood  seems  to  be  so  disordered  and  the  nutrition 
of  the  vessel- walls  so  altered  that  the  former  (even  some  of  the 
blood-cells)  passes  through  the  latter. 

Water. — The  use  of  water  certainly  has  a  great  influence 
over  the  metabolic  processes  of  the  body.  The  temporary  ad- 
dition or  withdrawal  of  even  a  few  ounces  of  water  from  the 
regular  supply  of  a  dog  in  the  course  of  a  feeding  experiment 
greatly  modfies  the  results  obtained  for  the  time.  It  is  well 
known  that  increase  of  water  in  the  diet  leads  to  a  correspond- 
ing increase  in  the  amount  of  urea  excreted.  It  is  likely  that 
even  yet  we  fail  to  appreciate  fully  the  great  part  which  water 
plays  in  the  animal  economy. 

THE   ENERGY  OF  THE   ANIMAL   BODY. 

As  already  explained,  we  distinguish  between  potential  or 
latent  and  actual  energy.  All  the  energy  of  the  body  is  to  be 
traced  to  the  influence  of  the  tissues  upon  the  food.  Energy 
may  be  estimated  as  mechanical  work  or  as  heat,  and  the  one 
may  be  converted  into  the  other.  All  the  processes  of  the 
organism  involve  chemical  changes,  and  a  large  proportion  of 
these  are  of  the  nature  of  oxidations ;  so  that  speaking  broadly, 
the  oxidations  of  the  animal  body  are  the  sources  of  its  energy ; 
and  in  estimating  the  quantity  of  energy,  either  as  heat  or  work, 
that  a  given  food-stuff  will  produce,  one  must  consider  whether 
the  oxidative  processes  are  complete  or  partial ;  thus,  in  the  case 
of  proteid  food,  if  we  suppose  that  the  urea  excreted  represents 
the  form  in  which  the  oxidative  processes  end  or  are  arrested, 
we  must,  in  estimating  the  actual  energy  of  the  proteid,  sub- 
tract the  amount  of  energy  that  would  be  produced  were  the 
urea  itself  completely  oxidized  (burned.) 


u± 


COMPARATIVE   PHYSIOLOGY. 


If  the  amount  of  heat  that  a  body  will  produce  in  its  com- 
bustion be  known,  then  by  the  law  of  the  conversion  and  equiv- 
alence of  energy  the  mechanical  equivalent  can  be  estimated  in 
that  particular  case. 

The  heat-producing  power  of  different  substances  can  be 
directly  learned  by  ascertaining  the  extent  to  which,  when  fully 
burned  (to  water  and  carbonic  anhydride),  they  elevate  the 
temperature  to  a  given  volume  of  water ;  and  this  can  at  once  be 
translated  into  its  mechanical  equivalent  of  work,  so  that  we 
may  say  that  one  gramme  of  dry  proteid  would  give  rise  to  a 
certain  number  of  gramme-degrees  of  heat  or  kilogramme- 
metres  of  work.  A  few  figures  will  now  show  the  relative 
values  of  certain  food -stuffs : 


1  gramme  proteid 

1  gramme  urea 

Available  energy  of  the  proteid 


Gram.-deg. 


5,103 
735 


4,308 


Kilomet. 


2,161 
311 


1,850 


The  reason  of  the  subtraction  has  been  explained  above. 

Taking  another  diet  in  regard  to  whicb  the  estimates  differ 
somewhat  from  those  given  previously,  but  convenient  now  as 
showing  how  equal  weights  of  substances  produce  very  dif- 
ferent amounts  of  energy,  we  find  that—. 


100  grammes  proteid  yield 
100  grammes  fat  yield  . . . 
240  grammes  starch  yield 

Total 


Gram.-deg. 


430,800 
900,900 
938,880 


2,281,580 


Kilomet. 


185,000 
384,100 
397,080 


900,780 


In  other  words  nearly  a  million  kilogramme-metres  of  en- 
ergy are  available  from  the  above  diet  for  one  day,  provided  it 
be  all  oxidized  in  the  body. 

Food-stuffs,  then,  with  the  oxygen  of  the  air,  are  the  body's 
sources  of  energy.  What  are  the  forms  in  which  its  expendi- 
ture appears  %  We  may  answer  at  once  heat  and  mechanical 
work ;  for  it  is  assumed  that  internal  movements  as  those  of 
the  viscera,  and  all  the  friction  of  the  body,  all  its  molcular 
motion,  all  secretive  processes,  are  to  be  regarded  as  finally 


THE  METABOLISM   OF   THE  BODY.  445 

augmenting  the  heat  of  the  hody.     Heat  is  lost  by  the  skin, 
lungs,  urine,  and  faeces. 

The  division  of  foods  into  heat-producers  and  tissue-builders 
is  unjustifiable,  as  will  appear  from  what  has  just  been  stated, 
as  well  as  from  such  facts  as  the  production  of  fat  from  proteid 
food,  thus  showing  that  the  latter  is  indirectly  a  producer  of 
carbonic  anhydride,  assuming  that  fat  is  oxidized  into  that 
substance. 

ANIMAL   HEAT. 

Though  a  large  part  of  the  heat  generated  within  the  body 
is  traceable  to  oxidations  taking  place  in  the  tissues,  it  is  better 
to  speak  of  the  heat  as  being  the  outcome  of  all  the  chemical 
processes  of  the  organism ;  and  though  heat  may  be  rendered 
latent  in  certain  organs  for  a  time,  in  the  end  it  must  appear. 
While  all  the  tissues  are  heat-producers  (thermogenic),  the  ex- 
tent to  which  they  are  such  would  depend,  we  should  suppose, 
upon  the  degree  to  which  they  were  the  seat  of  metabolic  pro- 
cesses ;  and  actual  tests  establish  this  fact.  Thus,  among  glands 
the  liver  is  the  greatest  heat-producer  ;  hence  the  blood  from 
this  organ  is  the  warmest  of  the  whole  body.  The  muscles  also 
are  especially  the  thermogenic  tissue. 

The  temperature  of  the  blood  in  the  hepatic  vein  is  wanner 
than  that  in  the  portal,  a  clear  evidence  that  the  metabolism  of 
this  organ  has  elevated  the  temperature  of  the  blood  flowing 
through  it. 

The  temperature  of  the  blood  (its  own  metabolism  being 
slight)  is  a  pretty  fair  indication  of  the  resultant  effect  of  the 
production  and  the  loss  of  heat. 

For  obvious  reasons,  the  temperature  of  different  parts  of 
the  body  of  man  and  other  animals  varies. 

The  statements  of  observers  in  regard  to  the  temperature  of 
various  animals  and  of  different  parts  of  the  body  disagree  in 
a  way  that  would  be  puzzling,  were  it  not  known  how  difficult 
it  is  to  procure  perfectly  accurate  thermometers,  not  to  mention 
individual  differences.  The  axillary  temperature  is  in  man 
about  37°  C.  (98  6  F.);  that  of  the  mouth  a  little  higher,  and 
of  the  rectum  or  vagina  slightly  more  elevated.  The  mean 
temperature  of  the  blood  is  placed  at  39°  C.  (102'2  F.). 

Comparative. — The  temperature  of  various  groups  of  animals 
has  been  stated  to  be  as  follows:  Hen  and  pigeon,  42°  (107'6  F.) ; 
swallow,  4403°  (111-25  F.) ;  dolphin,  35'50  (95  "9  F.) ;  mouse,  41-1° 


446  COMPARATIVE  PHYSIOLOGY. 

(106  F.);  snakes,  10°  to  12°  (50  to  53"6  F.) ;  but  higher  in  large 
specimens  (python).  Cold-blooded  animals  have  a  tempera- 
ture a  little  higher  (less  than  1°  C.  usually)  than  the  surround- 
ing air.  During  the  swarming  of  bees  the  hive  temperature 
may  rise  from  32°  to  40°  (89 -6  to  104  F.).  All  cold-blooded 
animals  have  probably  a  higher  temperature  in  the  breeding- 
season.  In  our  domestic  mammals  the  normal  temperature  is 
not  widely  different  from  that  of  man.  In  the  horse  the  aver- 
age is  37-5°  to  38°  (99 "5  to  100-4  F.) ;  in  the  ass,  38°  to  39-5°  (100-4 
to  103  F) ;  in  the  ox,  38°  to  38-5°  (100*4  to  101  "3  F.) ;  in  the  sheep 
and  pig,  39°  to  40°  (102-2  to  104  F.) ;  in  the  cat,  38'5°  to  39°  (101*3 
to  102-2  P.);  in  the  dog,  38-5°  (101-3  P.). 

Variations  in  the  average  temperature  are  dependent  on 
numerous  causes  which  may  affect  either  the  heat  production 
or  heat  loss  :  1.  Change  of  climate  has  a  very  slight  but  real 
influence,  the  temperature  being  elevated  a  fraction  of  a  degree 
when  an  individual  travels  from  the  poles  toward  the  equator, 
and  the  same  may  be  said  of  the  effect  of  the  temperature  of  a 
warm  summer  day  as  compared  with  a  cold  winter  one.  The 
wonder  is  that,  considering  the  external  temperature,  the  vari- 
ation is  so  light.  2.  Starvation  lowers  the  temperature,  and 
the  ingestion  of  food  raises  it  slightly,  the  latter  increasing,  the 
former  decreasing,  the  rate  of  the  metabolic  processes.  3.  Age 
has  its  influence,  the  very  young  and  the  very  old,  in  whom 
metabolism  (oxidation)  is  feeble,  having  a  lower  temperature. 
This  especially  applies  to  the  newly  born,  both  among  man- 
kind and  the  lower  mammals;  and,  as  might  be  supposed,  the 
temperature  falls  during  sleep,  when  all  the  vital  activi- 
ties are  diminished.  The  same  remark  applies  with  greater 
force  to  the  hibernating  state  of  animals.  The  temperature 
of  man  does  not  vary  more  than  about  1°  C.  during  the  twenty- 
four  hours. 

It  will  be  inferred,  from  the  facts  and  figures  already  cited, 
that  different  kinds  of  food  have  considerably  different  capacity 
for  heat  production. 

It  is  well  known  that  an  animal  when  working  not  only 
feels  warmer,  but  actually  produces  more  heat. 

It  appears  from  a  multitude  of  considerations  that  the  body 
is  like  a  steam-engine,  producing  beat  and  doing  work  ;  but  it 
is  found  that  while  a  very  good  steam-engine,  as  a  result  of  the 
chemical  processes  going  on  within  it,  converts  £  of  the  poten- 
tial energy  of  its  supplies  into  mechanical  work,  the  other  I 


THE  METABOLISM   OP   THE   BODY.  447 

appearing  as  heat,  the  animal  hody  produces  }  as  work  and  |  as 
heat,  from  its  income  of  food  and  oxygen. 

While  it  is  perfectly  clear  that  it  is  in  the  metabolic  pro- 
cesses of  the  body  that  we  must  seek  for  the  final  cause  of  the 
heat  produced,  it  is  incumbent  on  the  physiologist  to  explain 
the  remarkable  fact  that  the  mammalian  body  maintains, 
under  a  changing  external  temperature  and  other  climatic 
conditions,  and  with  a  varying  diet,  during  rest  and  labor,  a 
temperature  varying  within,  usually,  no  more  than  a  fraction 
of  a  degree  centigrade.  This  we  shall  now  endeavor  to  explain 
in  part. 

The  Regulation  of  Temperature.— It  is  manifest  from  the 
facts  adduced  that  so  long  as  life  lasts  heat  is  being  of  necessity 
constantly  produced.  If  there  were  no  provision  for  getting 
rid  of  a  portion  of  this  heat,  it  is  plain  that  the  body  would  soon 
be  consumed  as  effectually  as  if  it  were  placed  in  a  furnace. 
We  observe,  however,  that  heat  is  being  constantly  lost  by  the 
breath,  by  perspiration  (insensible),  by  conduction  and  radia- 
tion from  the  surface  of  the  body,  and  periodically  by  the 
urine  and  faeces.  We  have  seen  that,  while  heat  is  being  pro- 
duced in  all  the  tissues  and  organs  of  the  body,  some  are  es- 
specially  thermogenic,  as  the  glands  and  muscles.  The  skin 
presents  an  extensive  surface,  abundantly  supplied  with  blood- 
vessels, which  when  dilated  may  receive  a  large  quantity  of 
blood,  and  when  contracted  may  necessitate  a  much  larger  in- 
ternal supply,  in  the  splanchnic  region  especially.  It  is  a  mat- 
ter of  common  observation  that,  when  an  individual  exercises, 
the  skin  becomes  flushed,  and  so  with  the  increased  production 
of  heat,  especially  in  the  muscles  (see  page  195),  there  is  a  pro- 
vision for  unusual  escape  of  the  surplus  ;  at  the  same  time 
sweat  breaks  out  visibly,  or  if  not,  the  insensible  perspiration 
is  generally  increased  ;  and  this  accounts  for  an  additional  in- 
crement of  loss  ;  while  the  lungs  do  extra  work  and  exhale  an 
increased  quantity  of  aqueous  vapor,  so  that  in  these  various 
ways  the  body  is  cooled.  Manifestly  there  is  some  sort  of  rela- 
tion between  the  processes  of  heat  production  and  heat  expendi- 
ture. The  vaso-motor,  secretory,  and  respiratory  functions  are 
modified.  We  may  suppose  that  the  various  co-ordinations 
effected,  chiefly  at  all  events  through  the  central  nervous  sys- 
tem, and  not  by  the  direct  action  of  the  heat  upon  local  nerv- 
ous mechanisms,  or  the  tissues  themselves  directly,  are  re- 
flexes. 


448  COMPARATIVE   PHYSIOLOGY. 

The  'production  of  heat,  however,  seems  to  be  equally  under 
the  influence  of  the  nervous  system,  though  we  know  less  about 
the  details  of  the  matter. 

A  cold-blooded  animal  differs  from  a  warm-blooded  one  in 
that  its  temperature  varies  more  with  the  surrounding  medium: 
hence  the  terms  poikilothermer  and  homoiothermer  for  cold- 
blooded and  warm-blooded,  would  be  appropriate. 

Such  an  animal,  as  a  frog  or  turtle,  may  have  its  chemical 
processes  slowed  or  quickened,  almost  like  those  going  on  in  a 
test-tube  or  crucible,  by  altering  the  temperature.  Very  differ- 
ent is  it,  as  we  have  seen,  in  the  normal  state  of  the  animal  with 
any  mammal.  Hence  hibernation  or  an  allied  state  has  be- 
come a  necessary  protection  for  poikilothermers,  otherwise  they 
would  perish  outright,  and  the  groups  become  extinct  in  north- 
ern latitudes. 

It  is  plain  that  vaso-motor  changes  alone  can  not  explain 
these  effects  ;  and,  though  possibly  a  part  of  the  rise  of  tem- 
perature, following  exposure  of  the  naked  body  in  a  cool  air, 
may  be  accounted  for  by  the  increased  metabolism  of  internal 
organs,  accompanying  the  influx  of  blood  caused  by  constric- 
tion of  the  cutaneous  capillaries,  it  is  probable  that  in  this  as  in 
so  many  other  instances  the  blood  and  circulation  have  been 
credited  with  too  much,  and  the  direct  influence  of  the  nervous 
system  on  nutrition  and  heat  production  overlooked  or  under- 
estimated. The  thermogenic  center  has  not  yet  been  definitely 
located,  though  some  recent  investigations  seem  to  favor  a  spot 
in  or  near  the  corpus  striatum  for  certain  mammals.  Some  in- 
vestigators also  recognize  a  cortical  heat-center.  It  has  been 
suggested  that  we  may  to  advantage  speak  of  a  thermotaxic 
(regulative  of  loss)  and  a  thermogenic  mechanism  (regulative 
of  production),  and  even  a  thermolytic  or  discharging  mechan- 
ism. It  has  been  further  suggested  that  different  nerve-fibers 
may  be  concerned  in  the  actual  work  of  conveying  the  different 
impulses  of  these  respective  mechanisms  to  the  tissues ;  and  the 
whole  theory  has  been  framed  in  accordance  with  the  prevalent 
conception  of  metabolism  as  consisting  of  anabolism  and  ca- 
tabolism,  or  constructive  and  destructive  processes.  But  these 
theories  have  not  yet  been  confirmed  by  experiments  on  ani- 
mals, though  they  are,  in  the  opinion  of  their  authors,  in  har- 
mony with  the  facts  of  fever.  Certainly,  any  theory  that  will 
imply  that  vital  processes  are  more  under  the  control  of  the 
nervous  system  than  has  hitherto  been  taught,  will,  we  think, 


THE   METABOLISM   OF   THE   BODY.  449 

advance  physiology,  as  will  shortly  appear  from  our  discussion 
of  the  influence  of  the  nervous  system  on  the  various  metaholic 
processes  generally. 

The  phenomena  observable  in  an  animal  gradually  freezing 
to  death  point  strongly  to  the  direct  influence  of  the  nervous 
system  on  the  production  as  well  as  the  regulation  of  heat. 
The  circulation  must  of  course  be  largely  concerned,  but  it  ap- 
pears as  though  the  nervous  system  refused  to  act  when  the 
temperature  falls  below  a  certain  point.  A  low  temperature 
favors  hibernation, '  in  which  we  believe  the  nervous  system 
plays  the  chief  part,  though  the  temperature  in  itself  is  not  the 
determining  cause,  as  we  have  ourselves  proved.  The  fact  that 
the  whole  metabolism  of  a  hibernating  animal  is  lowered,  that 
with  this  there  is  loss  of  consciousness  much  more  profound 
than  in  ordinary  sleep,  of  itself  seems  to  indicate  that  the  nerv- 
ous system  is  at  the  bottom  of  the  whole  matter. 

Pathological, — It  is  found  that  many  drugs  and  poisons 
lower  temperature,  acting  in  a  variety  of  ways.  In  certain  dis- 
eases, as  cholera,  the  temperature  may  sink  to  23°  C.  in  extreme 
cases  before  death  supervenes.  When  the  temperatui'e  of  the 
blood  is  raised  6°  to  8°  C  (as  in  sunstroke,  etc.),  death  occurs  ; 
and  it  is  well  known  that  prolonged  high  tempei*ature  leads  to 
fatty  degeneration  of  the  tissues  generally.  All  the  evidence 
goes  to  show  that  in  fever  both  the  heat  production  and  the 
heat  expenditure  are  interfered  with  ;  or,  at  least,  if  not  always, 
that  there  may  be  in  certain  cases  such  a  double  disturbance. 
In  fever  excessive  consumption  of  oxygen  and  production  of 
carbon  dioxide  occur,  the  metabolism  is  quickened,  hence  its 
wasting  (consuming)  effects  ;  the  rapid  respiration  tends  to  in- 
crease the  thirst,  from  the  extra  amount  of  aqueous  vapor  ex- 
haled. The  body  is  actually  warmest  during  the  "  cold  stage  " 
of  fever,  when  the  vessels  of  the  skin  ai*e  constricted  and  the 
patient  feels  cold,  because  the  internal  metabolism  is  heightened ; 
while  the  "  sweating  stage  "  is  marked  by  a  natural  fall  of  tem- 
perature. The  fact  that  the  skin  may  be  dry  and  pale  in  fever 
shows  that  the  thermotaxic  nervous  mechanism  is  at  fault;  but 
the  chemical  facts  cited  above  (excess  of  CO2  etc.)  indicate  that 
the  thermogenic  mechanism  is  also  deranged. 
20 


450  COMPARATIVE   PHYSIOLOGY. 

SPECIAL   CONSIDERATIONS. 

If  the  student  will  now  read  afresh  what  has  been  written 
under  the  above  heading  in  relation  to  the  subject  of  digestion, 
it  will  probably  appear  in  a  new  light.  We  endeavored  to  show 
that,  according  to  that  general  principle  of  correlation  which 
holds  throughout  the  entire  organism,  and  in  harmony  with 
certain  facts,  we  were  bound  to  believe  that  digestion  and  as- 
similation, or,  to  speak  in  other  terms,  the  metabolic  processes 
of  the  various  tissues,  in  a  somewhat  restricted  sense,  were 
closely  related.  Beneath  the  common  observation  that  "  diges- 
tion waits  on  appetite  "  lies  the  deeper  truth  that  food  is  not 
prepared  in  the  alimentary  canal  (digested)  without  some  rela- 
tion to  the  needs  of  the  system  generally.  In  other  words,  the 
voice  of  the  tissues  elsewhere  is  heard  in  the  councils  of  the 
digestive  tract,  and  is  regarded  ;  and  this  is  effected  chiefly 
through  the  nervous  system.  Excess  in  eating  may  lead  to 
vomiting  or  diarrhoea — plain  ways  of  getting  rid  of  what  can 
not  be  digested. 

Evolution. — We  have  already  alluded  to  some  of  those  modi- 
fications in  the  form  of  the  digestive  organs  that  indicate  an 
unexpected  plasticity,  and  impress  the  fact  of  the  close  rela- 
tion of  form  and  function.  The  conversion  of  a  sea-gull  into  a 
graminivorous  bird,  with  a  corresponding  alteration  in  the  na- 
ture of  the  form  of  the  stomach  (it  becoming  a  gizzard),  with 
doubtless  modifications  in  the  digestive  processes,  when  re- 
garded more  closely,  implies  coadaptations  of  a  very  varied 
kind.  These  are  as  yet  but  imperfectly  known  or  understood, 
and  the  subject  is  a  wide  and  inviting  field  for  the  physiologist. 
Darwin  and  others  have  indicated,  though  but  imperfectly, 
some  of  the  changes  that  are  to  be  regarded  in  animals  as  cor- 
relations ;  but  in  physiology  the  subject  has  received  but  little 
attention  as  yet.  We  have  in  several  parts  of  this  work  called 
attention  to  it  ;  but  the  limits  of  space  prevent  us  doing  little 
more  than  attempting  to  widen  the  student's  field  of  vision  by 
introducing  such  considerations.  The  influence  of  climate  on 
metabolism,  an  undoubted  fact,  has  many  implications. 

Any  one  who  keeps  a  few  wild  animals  in  confinement  un- 
der close  observation,  and  endeavors  to  ascertain  how  their 
natural,  self-chosen  diet  may  be  varied  when  confined,  will 
be  astonished  at  the  plasticity  of  their  instincts,  usually  con- 
sidered as  so  rigid  in  regard  to  feeding.     These  facts  help  one 


THE   METABOLISM   OF  THE   BODY.  451 

to  understand  how  by  the  law  of  habit  and  heredity  each  group 
of  animals  has  come  to  prefer  and  flourish  best  upon  a  certain 
diet.  But  habit  itself  implies  an  original  deviation  some  time, 
in  which  is  involved,  again,  plasticity  of  nature  and  power  to 
adapt  as  well  as  to  organize.  Without  this,  evolution  of  func- 
tion is  incomprehensible  ;  but  with  this  principle,  and  the 
tendency  for  what  has  once  been  done  to  be  easier  of  repetition, 
and,  finally,  to  become  organized,  a  flood  of  light  is  thrown 
upon  the  subject  of  diet,  digestion,  and  metabolism  generally. 
On  these  principles  it  is  possible  to  understand  those  race  differ- 
ences, even  individual  differences,  which  as  facts  must  be  patent 
to  all  observers. 

The  principle  of  natural  selection  has  clearly  played  a  great 
part  in  determining  the  diet  of  a  species ;  the  surviving  immi- 
grants to  a  new  district  must  be  those  that  can  adapt  to  the  local 
environment  best,  including  the  food  which  the  region  supplies. 
The  greater  capability  of  resisting  hunger  and  thirst  in  some 
individuals  of  a  species  implies  great  differences  in  the  meta- 
bolic processes,  though  these  are  mostly  unknown  to  us;  and 
the  same  remark  applies  to  heat  and  cold 

It  seems  clear  that  hibernation  is  an  acquired  habit  of  the 
whole  metabolism,  with  great  changes  in  the  functional  condi- 
tion of  the  nervous  system  recurring  periodically,  and,  in  fact, 
dependent  on  these,  by  which  certain  large  divisions,  as  the 
reptiles,  amphibians,  and  certain  mammals  among  vertebrates, 
are  enabled  to  escape  individual  death  and  extinction  as  groups. 
We  may  suppose  that,  for  example,  among  invertebrates,  by  a 
process  of  natural  selection,  those  survived  that  could  thus  adapt 
themselves  to  the  environment ;  while,  among  mammals,  hiber- 
nation may  be  considered  as  a  process  of  reversion,  perhaps,  for 
the  homoiothermer  becomes  very  much  a  poikilothermer  during 
hibernation,  the  latter  reverting  to  a  condition  existing  in  lower 
forms,  and  not  wholly  unlike  that  of  plants  in  winter.  This 
can  be  understood  on  the  principle  of  the  origin  of  higher  from 
lower  forms;  otherwise  it  is  difficult  to  understand  why  similar 
states  of  the  metabolism  should  prevail  in  groups  widely  sepa- 
rated in  form  and  function.  If  all  higher  groups  bear  a  deriva- 
tive relation  to  the  lower,  what  is  common  in  their  nature,  as 
we  usually  find  them,  as  well  as  the  peculiar  resemblances  of 
the  metabolism  of  higher  to  lower  forms  in  sleep,  hibernation, 
etc.,  can  be  understood  in  the  light  of  physiological  reversion. 

The  origin  of  a  homoiothermic  (warm-blooded)   condition 


452  COMPARATIVE   PHYSIOLOGY. 

itself  is  to  be  sought  for  in  the  principle  of  natural  selection. 
It  was  open  to  certain  organisms,  we  may  assume,  either  to 
adapt  to  a  temperature  much  below  that  of  their  blood,  or  to 
hibernate;  failing  to  make  either  adaptation  would  result  in 
death;  and  gradually,  no  doubt,  involving  the  death  of  num- 
berless individuals  or  species,  the  resisting  power  attained  the 
marvelous  degree  that  we  are  constantly  witnessing  in  all 
homoiothermers. 

The  daily  variations  of  the  bodily  temperature  in  homoio- 
thermers is  a  beautiful  example  of  the  law  of  rhythm  evident 
in  the  metabolism.  Hibernation  is  another  such.  While  these 
are  clear  cases,  it  is  without  doubt  true  that,  did  we  but  know 
more  of  the  subject,  a  host  of  examples  of  the  operation  of  this 
law  might  be  instanced. 

We  can  but  touch  on  these  subjects  enough  to  show  that 
they  deserve  an  attention  not  as  yet  bestowed  on  them ;  and  to 
the  thoughtful  it  will  be  evident  that  their  influence  on  prac- 
tical life  might  be  made  very  great  were  they  but  rightly  ap- 
prehended. 

THE    INFLUENCE    OF   THE    NERVOUS    SYSTEM   ON 
METABOLISM  (NUTRITION). 

This  subject  is  of  the  utmost  importance,  and  has  not  re- 
ceived the  attention  hitherto,  in  works  on  physiology,  to  which 
we  believe  it  is  entitled,  so  that  we  must  discuss  it  at  some 
length. 

We  may  first  mention  a  number  of  facts  on  which  to  base 
conclusions :  1.  Section  of  the  nerves  of  bones  is  said  to  be  fol- 
lowed by  a  diminution  of  their  constituents,  indicating  an 
alteration  in  their  metabolism.  2.  Section  of  the  nerves  sup- 
plying a  cock's  comb  interferes  with  the  growth  of  that  ap- 
pendage. 3.  Section  of  the  spermatic  nerves  is  followed  by  de- 
generation of  the  testicle.  4.  After  injury  to  a  nerve  or  its 
center  in  the  brain  or  spinal  cord,  certain  affections  of  the 
skin  may  appear  in  regions  corresponding  to  the  distribution 
of  that  nerve ;  thus,  herpes  zoster  is  an  eruption  that  follows 
frequently  the  distribution  of  the  intercostal  nerve.  5.  When 
the  motor  cells  of  the  anterior  horn  of  the  spinal  cord  or  cer- 
tain cells  in  the  pons,  medulla,  or  crus  cerebri  are  disordered, 
there  is  a  form  of  muscular  atrophy  which  has  been  termed 
"  active,"  inasmuch  as  the  muscle  does  not  waste  merely,  but 


THE    METABOLISM   OE  THE  BODY.  453 

the  dwindling  is  accompanied  by  proliferation  of  the  muscle 
nuclei.  6.  After  neurotomy  for  navicular  disease  a  form  of  de- 
generation of  the  structures  of  the  foot  is  not  uncommon.  7. 
After  section  of  both  vagi,  death  results  after  a  period,  varying 
in  time,  as  do  also  the  symptoms  with  the  animal.  In  some 
animals  pneumonia  seems  to  account  for  death,  since  it  is 
found  that,  if  this  disease  be  prevented,  life  may,  at  all  events, 
be  greatly  prolonged.  The  pneumonia  has  been  attributed  to 
paralyses  of  the  muscles  of  the  larynx,  together  with  loss  of 
sensibility  of  the  larynx,  trachea,  bronchi,  and  the  lungs,  so 
that  the  glottis  is  not  closed  during  deglutition,  and  the  food, 
finding  its  way  into  the  lungs,  has  excited  the  disease  by  irrita- 
tion. The  possibility  of  vaso-motor  changes  is  not  to  be  over- 
looked. In  birds,  death  may  be  subsequent  to  pneumonia  or 
to  inanition  from  paralysis  of  the  oesophagus,  food  not  being 
swallowed.  It  is  noticed  that  in  these  creatures  there  is  fatty 
(and  sometimes  other)  degeneration  of  the  heart,  liver,  stomach, 
and  muscles.  8.  Section  of  the  trigeminus  nerve  within  the 
skull  has  led  to  disease  of  the  corresponding  eye.  This  opera- 
tion renders  the  whole  eye  insensible,  so  that  the  presence  of 
offending  bodies  is  not  recognized;  and  it  has  been  both  as- 
serted and  denied  that  protection  of  the  eye  from  these  pre- 
vents the  destructive  inflammation.  With  the  loss  of  sensi- 
bility there  is  also  vaso-motor  paralysis,  the  intra-ocular  ten- 
sion is  diminished,  and  the  relations  of  the  nutritive  lymph  to 
the  ocular  tissues  are  altered.  But  all  disturbances  of  the  eye 
in  which  there  are  vaso-motor  alterations  are  not  followed  by 
degenerative  changes.  9.  Degeneration  of  the  salivary  glands 
follows  suture  of  their  nerves.  10.  After  suture  of  long-di- 
vided nerves,  indolent  ulcers  have  been  known  to  heal  with 
great  rapidity.  This  last  fact  especially  calls  for  explanation. 
It  will  be  observed,  when  one  comes  to  examine  nearly  all  such 
instances  as  those  referred  to  above,  that  they  are  complex. 
Undoubtedly,  in  such  a  case  as  the  trigeminus  or  the  vagi, 
many  factors  contribute  to  the  destructive  issue;  but  the  fact 
that  many  symptoms  and  lesions  are  concomitants  does  not,  of 
itself,  negative  the  view  that  there  may  be  lesions  directly 
dependent  on  the  absence  of  the  functional  influence  of  nerve- 
Abel's.  We  prefer,  however,  to  discuss  the  subject  on  a  broader 
basis,  and  to  found  opinions  on  a  wider  survey  of  the  facts  of 
physiology. 

After  a  little  time  (a  few  hours),  when  the  nerves  of  the  sub- 


45i  COMPARATIVE  PHYSIOLOGY. 

maxillary  gland  have  been  divided,  a  flow  of  saliva  begins  and 
is  continuous  till  the  secreting  cells  become  altered  in  a  way 
visible  by  the  microscope.  Now,  we  have  learned  that  proto- 
plasm can  discharge  all  its  functions  in  the  lowest  forms  oi 
animals  and  in  plants  independently  of  nerves  altogether. 
What,  then,  is  the  explanation  of  this  so-called  u  paralytic  se- 
cretion "  of  saliva  ?  The  evidence  that  the  various  functions 
of  the  body  as  a  whole  are  discharged  as  individual  acts  or 
series  of  acts  correlated  to  other  functions  has  been  abundantly 
shown;  and,  looking  at  the  matter  closely,  it  must  seem  un- 
reasonable to  suppose  that  this  would  be  the  case  if  there  was 
not  a  close  supervision  by  the  nervous  system  over  even  the 
details  of  the  processes.  We  should  ask  that  the  contrary  be 
proved,  rather  than  that  the  burden  of  proof  should  rest  on  the 
other  side.  Let  us  assume  that  such  is  the  case ;  that  the  entire 
behavior  of  every  cell  of  the  body  is  directly  or  indirectly  con- 
trolled by  the  nervous  system  in  the  higher  animals,  especially 
mammals,  and  ask,  What  facts,  if  any,  are  opposed  to  such  a 
view  ?  We  must  suppose  that  a  secretory  cell  is  one  that  has 
been,  in  the  course  of  evolution,  specialized  for  this  end.  What- 
ever may  have  been  the  case  with  protoplasm  in  its  unspecialized 
form,  it  has  been  shown  that  gland-cells  can  secrete  independ- 
ently of  blood-supply  (page  314,  etc.)  when  the  nerves  going  to 
the  gland  are  stimulated.  Now,  if  these  nerves  have  learned,  in 
the  course  of  evolution,  to  secrete,  then  in  order  that  they  shall 
remain  natural  (not  degenerate)  they  must  of  necessity  secrete; 
which  means  that  they  must  be  the  subject  of  a  chain  of  meta- 
bolic processes,  of  which  the  final  link  only  is  the  expulsion  of 
formed  products.  Too  much  attention  was  at  one  time  directed 
to  the  latter.  It  was  forgotten,  or  rather  perhaps  unknown, 
that  the  so-called  secretion  was  only  the  last  of  a  long  series  of 
acts  of  the  cell.  True,  when  the  cells  are  left  to  themselves, 
when  no  influences  reach  them  from  the  stimulating  nervous 
centers,  their  metabolism  does  not  at  once  cease.  As  we  view 
it,  they  revert  to  an  original  ancestral  state,  when  they  pei'- 
formed  their  work,  lived  their  peculiar  individual  life  as  less 
specialized  forms  wholly  or  partially  independent  of  a  nervous 
system.  But  such  divorced  cells  fail;  they  do  not  produce 
normal  saliva,  their  molecular  condition  goes  wrong  at  once, 
and  this  is  soon  followed  by  departures  visible  by  means  of  the 
microscope.  But  just  as  secretion  is  usually  accompanied  by 
excess  of  blood,  so  most  functional  conditions,  if  not  all,  de- 


THE    METABOLISM   OF   THE  BODY.  455 

mand  an  unusual  supply  of  pabulum.  This  is,  however,  do 
more  a  cause  of  the  functional  condition  than  food  is  a  cause  of 
a  man's  working-.  It  may  hamper,  if  not  digested  and  assimi- 
lated. It  becomes,  then,  apparent  that  the  essential  for  metab- 
olism is  a  vital  connection  with  the  dominant  nervous  system. 

It  has  been  objected  that  the  nervous  system  has  a  metab- 
olism of  its  own  independent  of  other  regulative  influences; 
but  in  this  objection  it  seems  to  be  forgotten  that  the  nervous 
system  is  itself  made  up  of  parts  which  are  related  as  higher 
and  lower,  or  at  all  events  which  intercommunicate  and  ener- 
gize one  another.  We  have  learned  that  one  muscle-cell  has 
power  to  rouse  another  to  activity  when  an  impulse  has  reached 
it  from  a  nervous  center.  Doubtless  this  phenomenon  has 
many  parallels  in  the  body,  and  explains  how  remotely  a  nerv- 
ous center  may  exert  its  power.  It  enables  one  to  understand  to 
some  extent  many  of  those  wonderful  co-ordinations  (obscure 
in  detail)  that  are  constantly  taking  place  in  the  body.  We 
think  the  facts  as  they  accumulate  will  more  and  more  show, 
as  has  been  already  urged,  that  the  influence  of  blood-pressure 
on  the  metabolic  (nutritive)  processes  has  been  much  over- 
estimated. They  are  not  essential  but  concomitant  in  the 
highest  animals.  Turning  to  the  case  of  muscle  we  find  that 
when  a  skeletal  muscle  is  tetanized  the  essential  chemical  and 
electrical  phenomena  are  to  be  regarded  as  changes  differing  in 
degree  only  from  those  of  the  so-called  resting  state.  There  is 
more  oxygen  used,  more  carbonic  anhydride  excreted,  etc.  The 
change  in  form  seems  to  be  the  least  important  from  a  physio- 
logical point  of  view.  Now,  while  all  this  can  go  on  in  the 
absence  of  blood  or  even  of  oxygen,  it  can  not  take  place  with- 
out nerve  influence  or  something  simulating  it.  Cut  the  nerve 
of  a  muscle,  and  it  undergoes  fatty  degeneration,  and  atrophies. 
True,  this  may  be  deferred,  but  not  indefinitely,  by  the  applica- 
tion of  electricity,  acting  somewhat  like  a  nerve  itself,  and  in- 
ducing the  approximately  normal  series  of  metabolic  changes. 
If,  then,  the  condition  when  not  in  contraction  (rest)  differs 
from  the  latter  in  all  the  essential  metabolic  changes  in  rate  or 
degree  only  ;  and  if  the  functional  condition  or  accelerated 
metabolism  is  dependent  on  nerve  influence,  it  seems  reason- 
able to  believe  that  in  the  resting  condition  the  latter  is  not 
withheld. 

The  recent  investigations  on  the  heart  make  such  views  as 
we  are  urging'  clearer  still.     It  is  known  that  section  of  the 


456  COMPARATIVE   PHYSIOLOGY. 

vagi  leads  to  degeneration  of  the  cardiac  structure.  We  now 
know  that  this  nerve  contains  fibers  which  have  a  diverse 
action  on  the  metabolism  of  the  heart,  and  that,  according 
as  the  one  or  the  other  set  is  stimulated,  so  does  the  electri- 
cal condition  vary ;  and  everywhere,  so  far  as  known,  a  differ- 
ence in  electrical  conditions  seems  to  be  associated  with  a 
difference  in  metabolism,  which  may  be  one  of  degree  only, 
perhaps,  in  many  instances — still  a  difference.  The  facts  as 
brought  to  light  by  experimental  stimulation  harmonize  with 
the  facts  of  degeneration  of  the  cardiac  tissue  on  section  of  the 
vagi ;  but  this  is  only  clear  on  the  view  we  are  now  presenting, 
that  the  action  of  the  nervous  system  is  not  only  universal, 
but  that  it  is  constant ;  that  function  is  not  an  isolated  and 
independent  condition  of  an  organ  or  tissue,  but  a  part  of  a 
long  series  of  metabolic  changes.  It  is  true  that  one  or  more 
of  such  changes  may  be  arrested,  just  as  all  of  them  may  go 
on  at  a  less  rate,  if  this  actual  outpouring  of  pancreatic  secre- 
tion is  not  constant ;  but  secretion  is  not  summed  up  in  dis- 
charge merely ;  and,  on  the  other  hand,  it  would  seem  that  in 
some  animals  the  granules  of  the  digestive  glands  are  being 
renewed  while  they  are  being  used  up,  in  secreting  cells.  The 
processes  may  be  simultaneous  or  successive.  Nor  do  we  wish 
to  imply  that  the  nervous  system  merely  holds  in  check  or  in 
a  very  general  sense  co-ordinates  processes  that  go  on  unorigi- 
nated  "by  it.  We  think  the  facts  warrant  the  view  that  they  are 
in  the  highest  mammals  either  directly  (mostly)  or  indirectly 
originated  by  it,  that  they  would  not  take  place  in  the  absence 
of  this  constant  nervous  influence.  The  facts  of  common  ob- 
servation, as  well  as  the  facts  of  disease,  point  in  the  strongest 
way  to  such  a  conclusion.  Every  one  has  observed  the  in- 
fluence, on  not  one  but  many  functions  of  the  animal,  we  might 
say  the  entire  metabolism,  of  depressing  or  exalting  emotions. 
The  failure  of  appetite  and  loss  of  flesh  under  the  influence  of 
grief  or  worry,  tell  a  plain  story.  Such  broad  facts  are  of  infi- 
nitely more  value  in  settling  such  a  question  as  that  now  dis- 
cussed than  any  single  experiment.  The  best  test  of  any  theory 
is  the  extent  to  which  it  will  explain  the  whole  round  of  facts. 
Take  another  instance  of  the  influence  over  metabolism  of  the 
nervous  system. 

Every  trainer  of  race-horses  knows  that  he  may  overwork 
his  beast — i.  e. ,  he  may  use  his  muscles  so  much  as  to  disturb 
the  balance  of  his  powers  somewhere — very  frequently  his  di- 


THE   METABOLISM   OF   THE  BODY.  457 

gestion ;  but  often  there  seems  to  be  a  general  break — the  whole 
metabolism  of  the  body  seems  to  be  out  of  gear ;  and  the  same 
applies  to  our  domestic  animals.  If  we  assume  a  constant 
nervcus  influence  over  the  metabolic  processes,  this  is  compre- 
hensible. The  centers  can  produce  only  so  much  of  what  we 
may  call  nervous  force,  using  the  term  in  the  sense  of  directive 
power;  and  if  this  be  unduly  diverted  to  the  muscles,  other 
parts  must  suffer. 

On  this  view  also  the  value  of  rest  or  change  of  work 
becomes  clear.  The  neiwous  centers  are  not  without  some  re- 
semblance to  a  battery ;  at  most,  the  latter  can  generate  only  a 
definite  quantity  of  electricity,  and,  if  a  portion  of  this  be  di- 
verted along  one  conductor,  less  must  remain  to  pass  by  any 
other. 

It  is  of  practical  importance  to  recognize  that  under  great 
excitement  unusual  discharges  from  a  nerve-center  may  lead 
to  unwonted  functional  activity;  thus,  under  the  stimulus  of 
the  occasion  an  animal  may  in  a  race  originate  muscular  con- 
tractions that  he  could  not  call  forth  under  other  circumstances. 
Such  are  always  dangerous.  We  might  speak  of  a  reserve  or 
residualnerve  force,  the  expenditure  of  which  results  iu  serious 
disability. 

It  seems  that  the  usually  taught  views  of  secretion  and 
nutrition  have  been  partial  rather  than  erroneous  in  themselves, 
and  it  is  a  question  whether  it  would  not  be  well  to  substitute 
some  other  terms  for  them,  or  at  least  to  recognize  them  more 
clearly  as  phases  of  a  universal  metabolism.  We  appear  to  be 
warranted  in  making  a  wider  generalization.  To  regard  pro- 
cesses concerned  in  building  up  a  tissue  as  apart  from  those  that 
are  recognized  as  constituting  its  function,  seems  with  the  knowl- 
edge we  at  present  possess,  to  be  illogical  and  unwise.  Whether, 
in  the  course  of  evolution,  certain  nerves,  or,  as  seems  more 
likely,  certain  nerve-fibers  in  the  body  of  nerve-trunks,  have 
become  the  medium  of  impulses  that  are  restricted  to  regulat- 
ing certain  phases  of  metabolism — as  e.  g.,  expulsion  of  formed 
products  in  gland-cells — is  not,  from  a  general  point  of  view, 
improbable,  and  is  a  fitting  subject  for  fui'ther  investigation. 
But  it  will  be  seen  that  we  should  regard  all  nerves  as  "  tro- 
phic "  in  the  wider  sense.  What  is  most  needed,  apparently,  is 
a  more  just  estimation  of  the  relative  parts  played  by  blood 
and  blood-  pressure,  and  the  direct  influence  of  the  nervous 
system  on  the  life-work  of  the  cell. 


458  COMPARATIVE   PHYSIOLOGY. 

We  must  regard  the  nervous  centers  as  the  source  of  cease- 
less impulses  that  operate  upon  all  parts,  originating  and  con- 
trolling the  entire  metabolism,  of  which  what  we  term  func- 
tions are  but  certain  phases,  parts  of  a  whole,  but  essential  for 
the  health  or  normal  condition  of  the  tissues.  Against  such  a 
view  we  know  no  facts,  either  of  the  healthy  or  disordered  or- 
ganism. 

Summary  of  Metabolism.— Very  briefly  and  somewhat  in- 
completely, we  may  sum  up  the  chief  results  of  our  present 
knowledge  (and  ignorance)  as  follows: 

Glycogen  is  found  in  the  livers  of  all  vertebrate  and  some 
invertebrate  animals.  The  quantity  varies  with  the  diet,  being 
greatest  with  an  excess  of  carbohydrates. 

Glycogen  may  be  regarded  as  stored  material  to  be  convert- 
ed into  sugar,  as  required  by  the  organism ;  though  the  exact 
use  of  the  sugar  and  the  method  of  its  disposal  are  unknown. 

Fat  is  not  stored  up  in  the  body  as  the  result  of  being 
merely  picked  out  from  the  blood  ready  made ;  but  is  a  genuine 
product  of  the  metabolism  of  the  tissues,  and  may  be  formed 
from  fatty,  carbohydrate,  or  proteid  food.  This  becomes  es- 
pecially clear  when  the  difference  in  the  fat  of  animals  from 
that  on  which  they  feed  is  considered,  as  well  as  the  direct  re- 
sults of  feeding  experiments,  and  the  nature  of  the  secretion  of 
milk. 

The  liver  seems  to  be  engaged  in  a  very  varied  round  of  meta- 
bolic processes  ;  the  manufacture  of  bile,  of  glycogen,  of  urea, 
and  probably  of  many  other  substances,  some  known  and 
others  unknown,  as  chemical  individuals.  Urea  is  in  great 
part  probably  only  appropriated  by  tbe  kidney-cells  (Amoeba- 
like) from  the  blood  in  which  it  is  found  ready  made ;  though 
it  may  be  that  a  part  is  formed  in  these  cells,  either  from 
bodies  some  steps  on  the  way  toward  urea,  or  out  of  their  pro- 
toplasm, as  fat  seems  to  be  by  the  cells  of  the  mammary  gland. 

The  leucin  (and  tyrosin  ?)  of  the  digestive  canal  sustains 
some  relation  to  the  manufacture  of  urea  by  the  liver,  and  pos- 
sibly by  tbe  spleen  and  other  organs ;  for  a  proteid  diet  increases 
these  products,  and  also  the  urea  excreted.  Creatin,  one  of  the 
products  of  proteid  metabolism,  and  possibly  allied  bodies,  may 
be  considered  as  in  a  certain  sense  antecedents  of  urea ;  uric-acid, 
however,  does  not  seem  to  be  such,  nor  is  it  to  be  regarded  as  a 
body  that  has  some  of  it  escaped  complete  oxidation,  but  rather 
as  a  result  of  a  distinct  departure  of  the  metabolism  ;  and  there 


THE  METABOLISM   OF   THE   BODY.  459 

are  facts  which  seem  to  indicate  that  the  uric-acid  metabolism 
is  the  older,  from  an  evolutionary  point  of  view,  and  that  in 
mammals,  and  especially  in  man,  as  the  results  of  certain  errors 
there  may  be  a  physiological  (or  pathological)  reversion.  Hip- 
puric  acid,  as  replacing  uric  acid  in  the  herbivora,  maybe  re- 
garded in  a  similar  light. 

Our  knowledge  of  the  metabolism  of  the  spleen,  beyond  its 
relations  to  the  formation  of  blood-cells  and  their  disintegra- 
tion, is  in  the  suggestive  rather  than  the  positive  stage.  It 
seems  highly  probable  that  this  organ  plays  a  very  important 
part,  the  exact  nature  of  which  is  as  yet  unknown. 

When  an  animal  starves,  it  may  be  considered  as  feeding  on 
its  own  tissues,  the  more  active  and  important  utilizing  the 
others.  Notwithstanding,  organs  with  a  very  active  metabo- 
lism, as  the  muscles  and  glands,  lose  weight  to  a  large  extent. 
The  presence  of  urea  to  an  amount  not  very  greatly  below  the 
average  in  health,  shows  that  there  is  an  active  proteid  metabo- 
lism then  as  at  all  times  in  progress. 

General  experience  and  exact  experiments  prove  that,  while 
an  animal's  diet  may  be  supplied  with  special  regard  to  fatten- 
ing, to  increase  working  power,  or  simply  to  maintain  it  in 
health,  as  evidenced  by  breeding  capacity,  form,  etc.,  in  all 
cases  there  must  be  at  least  a  certain  minimum  quantity  of  each 
of  the  food-stuffs.  No  one  food  can  be  said  to  be  exclusively 
fattening,  heat-forming,  or  muscle-forming. 

A  carbohydrate  diet  tends  to  production  of  fat ;  proteid  food 
to  supply  muscular  energy,  but  the  latter  also  produces  fat,  and 
a  diet  of  proteid  mixed  with  fat  or  gelatin  will  serve  the  pur- 
poses of  the  economy  better  than  one  containing  a  very  much 
larger  quantity  of  proteid  alone.  Muscular  energy,  as  is  to  be 
inferred  from  the  excreta,  is  not  the  result  of  nitrogenous  me- 
tabolism alone;  and  in  arranging  any  diet  for  man  or  beast  the 
race  and  the  individual  must  be  considered.  Animals  can  not 
be  treated  as  machines,  like  engines  using  similar  quantities  of 
fuel  ;  though  this  holds  far  more  of  man  than  the  lower  ani- 
mals— i.e.,  the  results  may  be  predicted  from  the  diet  with  far 
more  certainty  in  their  case  than  for  man. 

Food  is  related  to  excreta  in  a  definite  way,  so  that  all  that 
enters  as  food  must  sooner  or  later  appear  as  urea,  salts,  car- 
bonic anhydride,  water,  etc.  These  are  individually  to  be  re- 
garded as  the  final  links  in  a  long  chain  of  metabolic  processes, 
or  rather  a  series  of  these.     Fats  and  carbohydrates  are  repre- 


460  COMPARATIVE  PHYSIOLOGY. 

sented  finally  as  carbonic  anhydride  and  water  principally, 
proteids  as  urea. 

Nitrogenous  foods  may  be  regarded  as  accelerating  the 
metabolic  processes  generally  and  proteid  metabolism  in  par- 
ticular, while  fats  have  the  reverse  effect ;  hence  fat  in  the  diet 
renders  a  less  quantity  of  proteid  sufficient.  Gelatin  seems  to 
act  when  mixed  with  proteid  food  either  like  an  additional 
quantity  of  proteid,  or  possibly  like  fat,  at  all  events  under  such 
circumstances  less  proteid  suffices. 

These  facts  have  a  bearing  not  only  on  health  but  on  econ- 
omy, in  the  expenditure  for  food. 

Salts  hold  a  very  important  place  in  every  diet,  though 
their  exact  influence  is  in  great  part  unknown.  The  heat  of 
the  body  is  the  resultant  of  all  'the  metabolic  processes  of  the 
organism,  especially  the  oxidative  ones.  Certain  food-stuffs 
have  greater  potential  capacity  for  heat  formation  than  others  ; 
but,  finally,  the  result  depends  on  whether  the  organism  can 
best  utilize  one  or  the  other. 

A  certain  body  temperature,  varying  only  within  narrow 
limits,  is  maintained,  partly  by  regulation  of  the  supply  and 
partly  by  the  regulation  of  the  loss. 

Both  these  are,  in  health,  under  the  direction  of  the  nervous 
system,  and  both  are  co-ordinated  by  the  same.  Loss  is  chiefly 
through  the  skin  and  lungs  ;  gain  chiefly  through  the  organs 
of  most  active  metabolism,  as  the  muscles  and  glands. 

Vaso-motor  effects  play  a  great  part  in  the  escape  of  heat. 

Animals  may  be  divided  into  poikilothermers  and  homoio- 
thermers,  or  cold-blooded  and  warm-blooded  animals,  accord- 
ing as  their  body  heat  varies  with  or  is  independent  of  the  ex- 
ternal changes  of  temperature.  All  the  facts  go  to  show  that 
in  mammals  the  processes  of  the  body  (metabolism)  can  con- 
tinue only  within  a  slight  range  of  variations  in  temperature, 
though  the  upward  limit  is  narrower  than  the  downward. 

Upon  the  whole,  the  evidence  justifies  the  conclusion  that 
the  nervous  system  is  concerned  in  all  the  metabolic  processes 
of  the  body  in  mammals  including  man,  and  that,  as  we  descend 
the  scale,  the  dominion  of  the  nervous  system  becomes  less  till 
we  reach  a  point  when  protoplasm  goes  through  the  whole 
cycle  of  its  changes  by  virtue  of  its  own  properties  uninfluenced 
by  any  modification  of  itself  in  the  form  of  a  nervous  system. 


THE   SPINAL   CORD.— GENERAL. 


Among  the  higher  vertebrates  the  spinal  cord  is  found  to 
consist  of  nerve-cells,  nerve-fibers,  and  a  delicate  connective  tis- 
sue binding  them  together  ;  while  these  different  structures  are 
arranged  in  definite  forms,  so  that  a  cross-section  anywhere  pre- 
sents a  characteristic  appearance,  the  more  important  gangli- 
onic nerve-cells  being  internal  and  forming  a  large  part  of 
the  gray  matter  of  the  cord.  All  the  various  regions  of  this 
organ  or  series  of  organs  are  connected  with  one  another, 
white  with  white  and  gray  matter,  as  well  as  white  with  gray 
substance. 

While  we  do  not  attempt  to  furnish  a  complete  and  detailed 
account  of  the  anatomy  of  the  cord  or  other  parts  of  the  nervous 
system,  for  which  the  student  is  referred  to  works  on  anatomy, 
we  would  remind  him  that  the  spinal  cord  is  situated  within-  a 
bony  case  with  joints  permitting  of  a  certain  amount  of  move- 
ment, variable  in  different  regions.  Inasmuch  as  the  cord  itself 
does  not  fill  its  bony  covering,  but  floats  in  fluid  and  tethered 
to  the  walls  by  bands  of  connective  tissue,  it  is  well  protected 
from  laceration,  bruising,  or  concussion.  Like  the  brain,  it  has 
a  protective  tough  outer  membrane  (dura  mater)  with  a  closer- 
fitting  iuner  covering  abounding  in  blood-vessels  (pia  mater). 

The  white  matter  of  the  cord  invests  the  horns  of  gray 
matter  and  is  made  up  of  nerve-fibers  wanting  the  outer  sheath. 
Here,  as  elsewhere,  these  fibers  have  only  a  conducting  func- 
tion ;  they  do  not  originate  nervous  impulses.  The  gray  matter, 
on  the  other  hand,  abounds  in  cells,  some  of  them  with  many 
processes,  that  can  originate,  modify,  and  conduct  impulses. 
Certain  well-recognized  groups  of  these  cells  are  arranged  in 
columns  throughout  the  cord,  as  shown  in  the  accompany- 
ing figures.  The  supporting  basis  for  these  cells  (neuroglia)  is 
the  most  delicate  form  of  connective  tissue  known. 

The  cord  may  be  regarded  either  as  an  instrument  for  the 


Fio.  32' 


Fio.  328. 


THE  SPINAL   CORD.— GENERAL. 


463 


Fig.  327.— General  view  of  spinal  cord  (Chauveau).  A,  cervical  bulb;  B,  lumbar  bulb; 
C,  cauda  equina. 

Fig.  328.— Segment  of  spinal  cord  at  the  cervical  bulb,  or  brachial  plexus,  showing  its 
upper  face  and  the  roots  of  the  spinal  nerves  (Chauveau).  A,  superior  roots;  J5, 
inferior  roots;  C,  multiple  ganglia  of  superior  roots;  D,  single  ganglion  on  an 
exceptional  pair;  E,  £,  upper  roots  passing  through  the  envelopes. 

reception  and  generation  of  impulses  independent  of  the  brain ; 
or  as  a  conductor  of  afferent  and  efferent  impulses  destined  for 
the  brain  or  originating  in  that  organ.  As  a  matter  of  fact, 
however,  it  is  better  to  bear  in  mind  that  the  cord  and  brain 
constitute  one  organ  or  chain  of  organs,  which,  as  we  have 
learned  from  our  studies  in  development,  are  differentiations 
of  one  common  track,  originating  from  the  epiblast. 

While  the  brain  and  the  cord  may  act  independently  to  a 


Fig.  329.— Transverse  section  of  spinal  cord  of  child  six  months  old,  at  middle  of  lum- 
bar region,  showing  especially  the  fibers  of  gray  substance.  1  x  20.  (After  Ger- 
lach.)  a,  anterior  columns;  b.  posterior  columns;  c,  lateral  columns:  <l.  anterior 
roots;  e,  posterior  roots;  f,  anterior  white  commissure;  r/,  central  canal  lined  by 
epithelial  cells;  h,  connective-tissue  substance  surrounding  it;  i.  transverse  fibers 
of  gray  commissure  in  front,  and  k,  the  same  behind  central  canal:  /.  I  wo  veins 
cut  across:  m.  anterior  eornua:  it,  great  lateral  cell  group  of  anterior  cornua;  o, 
lesser  anterior  cell  group  (column):  px  smallest  median  cell  group;  q,  posterior 
cornua;n  ascending  fasciculi  in  posterior  cornua;  *',  substantia  gelatinosa. 


464 


COMPARATIVE   PHYSIOLOGY. 


Fig.  330.— Group  of  cells  in  connection  with  anterior  roots  of  spinal  nerves,  as  seen  in 
transverse  section  of  spinal  cord  of  sheep  (after  Flint  and  Dean).  A,  emergence 
of  anterior  roots  from  gray  matter;  b,  b,  b,  cells  connected  both  with  each  other 
and  with  fibers  of  anterior  roots. 

very  large  extent,  as  may  be  shown  by  experiment,  yet  it  can 
not  be  too  well  borne  in  mind  that  in  the  actual  normal  life  of 
an  animal  such  purely  independent  behavior  must  be  exceed- 
ingly rare.  We  are  constantly  in  danger,  in  studying  a  sub- 
ject, of  making  in  our  minds  isolations  which  do  not  exist  in 
nature.     When  one  accidentally  sits  upon  a  sharp  object,  he 


THE  SPINAL  CORD.— GENERAL. 


465 


Fig.  331.— Division  of  a  slender  nerve-fiber,  and  communication  of  its  branches  with 
highly  ramifying  processes  of  two  nerve-cells  from  spinal  cord  of  ox.  1  x  150. 
(After  Gerlach.) 

rises  suddenly  without  a  special  effort  of  will  power;  he  expe- 
riences pain,  and  has  certain  thoughts  about  the  object,  etc. 
30 


466 


COMPARATIVE  PHYSIOLOGY. 


Now,  in  reality  this  is 
very  complex,  though  it 
can  be  analyzed  into  its 
factors.  Thus,  afferent 
nerves  are  concerned,  the 
spinal  cord  as  a  reflex 
center,  efferent  nerves  to 
the  muscles  called  into 
action,  the  cord  as  a  con- 
ductor of  impulses  which 
result  in  sensations,  emo- 
tions, and  thoughts  refer- 
able to  the  brain ;  so  that 
if  we  would  grasp  the  state 
of  affairs  it  is  of  impor- 
tance to  so  combine  the 
various  processes  in  our 
mental  conception  that  it 
shall  in  our  minds  form 
that  whole  which  corre- 
sponds with  nature,  as  we 
have  been  insisting  upon 
in  the  last  chapter.  With 
this  admonition,  and  as- 
suming a  good  knowledge 
of  the  general  and  minute 

Fig.  332.— Multipolar  ganglion  cell  from  anterior  ,  »      ,-,  ■, 

gray  matter  of  spinal  cord  of  ox  (after  Dei-  anatomy  OI  Uie  spinal 
ters).  a,  axis  cylinder  process;  b,  branched  cor(]  we  shall  nrc-ceed  to 
processes.  -  " 

discuss  its  functions. 


THE   REFLEX   FUNCTIONS   OF  THE   SPINAL   CORD. 

The  following  experimental  observations  may  readily  be^ 
made  by  the  student  himself:  Let  a  decapitated  frog  be  sus- 
pended freely  (from  the  lower  jaw).  It  hangs  motionless  and 
limp  at  first,  but  when  it  recovers  from  the  shock  (abolition  of 
function)  to  the  spinal  cord  produced  by  the  operation,  it  may 
be  shown  that  this  organ  is  functional:  1.  When  a  piece  of 
bibulous  paper  dipped  in  dilute  acid  is  placed  upon  the  thigh, 
the  leg  is  drawn  up  and  wipes  away  the  offending  body.  2.  If 
the  paper  be  placed  on  the  anus,  both  legs  may  be  drawn  up, 
either  successively  or  simultaneously.     3.  If  the  leg  of   one 


THE   SPINAL  CORD.— GENERAL.  467 

side  be  allowed  to  hang  in  the  dilute  acid,  it  will  he  withdrawn. 
4.  If  a  small  piece  of  blotting-paper  dipped  in  the  acid  be 
placed  on  the  thigh,  and  the  leg  of  that  side  gently  held,  the 
other  may  be  drawn  up  and  remove  the  object. 

It  may  be  noticed  that  in  every  case  a  certain  interval  of 
time  elapses  before  the  result  follows.  Upon  increasing  the 
strength  of  the  acid  very  much  this  interval  is  shortened,  and 
the  number  of  groups  of  muscles  called  into  action  is  increased. 
Again,  the  result  is  not  the  same  in  all  respects  when  the  nerve 
of  the  leg  is  directly  stimulated,  as  when  the  skin  first  receives 
the  impression.  Section  of  the  nerves  of  the  parts  abolishes 
these  effects;  so  also  does  destruction  of  the  spinal  cord,  or  the 
part  of  it  with  which  the  nerves  of  the  localities  stimulated  are 
connected ;  and  more  exact  experiments  show  that  in  the  ab- 
sence of  the  gray  matter  the  section  of  the  posterior  or  anterior 
roots  of  the  nerves  also  renders  such  manifestations  as  we  have 
been  describing  impossible. 

These  experiments  and  others  seem  to  show  that  an  afferent 
nerve,  an  efferent  nerve,  and  one  or  more  central  cells  are 
necessary  for  a  reflex  action ;  that  the  latter  is  only  a  perfectly 
co-ordinated  one  when  the  skin  (end-organs)  and  not  the  nerve- 
trunks  are  stimulated ;  that  there  is  a  latent  period  of  stimula- 
tion, suggesting  a  central  "  summation  "  of  impulses  necessary 
for  the  effect  ;  that  the  reflex  is  not  due  to  the  mere  passage  of 
impulses  from  an  afferent  to  an  efferent  nerve  through  the 
cord,  but  implies  important  processes  in  the  central  cells  them- 
selves. The  latter  is  made  further  evident  from  the  fact  that 
(1)  strychnia  greatly  alters  reflex  action  by  shortening  the 
latent  period  and  extending  the  range  of  muscular  action,  which, 
it  has  been  shown,  is  not  due  to  changes  in  the  nerves  them- 
selves. A  very  slight  stimulus  suffices  in  this  instance  to  cause 
the  whole  body  of  a  decapitated  frog  to  pass  into  a  tetanic 
spasm.  We  must  suppose  that  the  processes  usually  confined 
to  certain  groups  of  central  cells  have  in  such  a  case  involved 
others,  or  that  the  "  resistance  "  of  the  centers  of  the  cord  has 
been  diminished,  so  that  many  more  cells  are  now  involved; 
hence  many  more  muscles  called  into  action.  Normally  there  is 
resistance  to  the  passage  of  an  impulse  to  the  opposite  side  of  the 
cord,  as  is  shown  by  the  fact  that  when  a  slight  stimulus  is  ap- 
plied to  the  leg  of  one  side  the  reflex  is  confined  to  this  member. 

It  is  evident,  then,  that  the  reflex  resulting  is  dependent  on 
(1)  the  location  of  the  stimulus,  (2)  its  intensity  and  duration. 


46  S 


COMPARATIVE   PHYSIOLOGY. 


(3)  its  character,  and  (4)  the  condition  of  the  spinal  cord  at  the 
time.  Occasionally  on  irritating  one  fore-limb  the  opposite 
hind  one  answers  reflexly.  Such  is  a  "'  crossed  reflex,"  and  is 
the  more  readily  induced  in  animals  the  natural  gait  of  which 
involves  the  use  of  one  fore-leg  and  the  opposite  hind-limb 
together. 


MOTOR 

BRANCH 


SENSORY? 

bbucs 

Fig.  333.— Diagram  of  a  spinal  segment  showing  component  parts  (Ranney). 


Fig.  334.— Diagrammatic  representation  to  illustrate  the  reflex  arc  (Bramwell  and  Ran- 
ney). 1,  2,  sensory  fibers;  3,  motor-cell  of  anterior  horn;  4,  motor-fiber  connected 
with  3  and  passing  out  by  anterior  root  to  muscle;  5,  fiber  joining  ganglionic  cell 
(3)  with  crossed  pyramidal  tract,  C.  P.  C;  G,  ganglion  on  root  of  posterior  spinal 
nerve;  7,  fiber  joining  3  with  Torek's  column,  T.  Fiber  2  is  represented  as  pass- 
ing through  Burdach's  column  to  reach  the  cell,  3. 

Reflexes  are  often  spoken  of  as  purposive,  and  suggest  at 
first  intelligence  in  the  cord;  but  such  phenomena  are  explained 
readily  enough  without  such  a  strained  assumption. 

Evolution,  heredity,  and  the  law  of  habit,  apply  here  as  else- 
where. The  relations  of  an  animal  to  its  environment  must 
necessarily  call  into  play  certain  nervo-muscular  mechanisms, 


THE  SPINAL  CORD.— GENERAL.  469 

which  from  the  law  of  habit  come  to  act  together  when  a 
stimulus  is  applied.  Naturally  those  that  make  for  the  welfare 
of  the  animal  are  such  as  are  most  used  under  the  influence  of 
the  intelligence  of  the  animal — i.  e.,  of  the  domination  of  the 
higher  cerebral  centers,  so  that  when  the  latter  are  removed  it 
is  but  natural  that  the  old  mechanisms  should  be  still  employed. 
Moreover,  the  reflex  movements  are  not  always  beneficial,  as 
when  a  decapitated  snake  coils  itself  around  a  heated  iron 
under  reflex  influence,  which  is  readily  enough  understood  if 
we  remember  the  habit  of  coiling  around  objects,  and  what 
this  involves— viz.,  organized  tendencies. 

Inhibition  of  Reflexes. — It  can  be  shown  in  the  case  of  a  frog 
that  still  retains  its  optic  lobes  and  the  parts  of  the  brain  pos- 
terior to  them  that,  when  these  are  stimulated  at  the  same  time 
as  the  leg,  the  reflex,  if  it  occurs  at  all,  is  greatly  delayed. 

On  the  other  hand,  in  the  case  of  dogs,  from  which  a  part 
of  the  cerebral  cortex  has  been  removed,  the  reflexes  are  much 
more  prominent  than  before.  Experience  teaches  us  that  the 
acts  of  defecation,  micturition,  erection  of  the  penis,  and  many 
others,  are  susceptible  of  arrest  or  may  be  prevented  entirely 
when  the  usual  stimuli  are  still  active,  by  emotions,  etc. 

These  and  numerous  other  facts  tend  to  show  that  the  higher 
centers  of  the  brain  can  control  the  lower;  and  it  is  not  to  be 
doubted  that  pure  reflexes  during  the  waking  hours  of  the 
higher  animals,  and  especially  of  man,  are  much  less  numerous 
than  among  the  lower  vertebrates.  The  cord  is  the  servant  of 
the  brain,  and  a  faithful  and  obedient  one,  except  in  cases  of 
disease,  to  some  forms  of  which  we  have  already  referred. 

THE    SPINAL  CORD   AS   A   CONDUCTOR   OF  IMPULSES. 

It  is  to  be  carefully  borne  in  mind  now,  and  when  studying 
the  brain,  that  a  conducting  path  in  the  nervous  centers  is  not 
synonymous  with  conducting  fibers.  The  cells  themselves 
and  the  neuroglia  probably  are  also  conductors.  We  shall 
now  endeavor  to  map  out,  as  established  by  the  method  of 
Flechsig,  Waller,  and  others,  the  main  fiber  tracts  of  the  spinal 
cord. 

1.  Antero-median  Columns  (columns  of  Turck). — These 
probably  decussate  in  the  cervical  region,  where  they  are  most 
marked,  constituting  the  direct  or  uncrossed  pyramidal  tract 
and  disappear  in  the  lower  dorsal  region. 


470 


COMPARATIVE   PHYSIOLOGY. 


PR< 


Secondary  degeneration  ensues  in  these  tracts  upon  certain 

brain  lesions,  in  the  motor  regions. 

2.  Crossed  Pyramidal  Tracts. — They  pass  forward  to  form 

part  of  the  anterior  pyramids  of  the  medulla  after  decussation 

in  their  lower  part.  Simi- 
larly to  the  first,  degenera- 
tion follows  in  these  tracts 
when  there  are  brain  -  le- 
sions of  the  motor  area. 
Hence,  both  of  these  consti- 
tute descendingmotor  paths. 

3.  Anterior  Fascicidi 
(fundamental  or  ground 
bundle).  —  They  possibly 
connect  the  gray  matter  of 
the  cord  with  that  of  the 
medulla. 

4.  Anterior  Radicular 
Zones,  in  the  anterior  part 
of  the  lateral  column. 

5.  Mixed  Lateral  Col- 
umns.— These  and  the  pre- 

Fig.  335.— Diagrammatic  representation  of  col-  _  . 

umns  and  conducting  paths  in  spinal  cord  ceding  are  functionally  sim- 
in  upper  dorsal  region  (after  Flint  and 
Landois).  AR.  AR,  anterior  roots  of  spi- 
nal nerves;  PR,  PR,  posterior  roots;  A, 
columns  of  Ttirck  (antero-  median  col- 
umns) ;  B,  anterior  fundamental  fascicu- 
lus; C,  columns  of  Goll;  D.  columns  of 
Burdach;  E,  E,  anterior  radicular  zones; 
F,  F,  mixed  lateral  columns;  G,  G,  crossed 
pyramidal  tracts;  II,  II,  direct  cerebellar  trophic  cells  both  above  and 
fibers.  t     t 

below. 

6.  Direct  Cerebellar  Tracts. — These  bundles,  passing  by  the 
funiculi  graciles  or  posterior  pyramids  of  the  medulla,  reach 
the  cerebellum  by  its  inferior  peduncles. 

These  fasciculi  enlarge  from  their  site  of  origin  in  the  lum- 
bar cord  upward.  After  section  of  the  cord  they  show  ascend- 
ing degeneration,  so  that  it  seems  probable  that  their  trophic 
cells  are  to  be  referred  to  the  posterior  gray  cornua  of  the  cord, 
which  they  connect  in  all  probability  with  the  cerebellum. 

7.  Columns  of  Burdach  (postero-lateral  columns). — This 
tract  is  connected  with  the  restiform  bodies  and  reaches  the 
cerebellum  by  the  inferior  peduncles.  Secondary  degenera- 
tions do  not  occur  in  these  fasciculi,  so  that  it  seems  likely  that 
they  connect  nerve-cells  at  different  levels  in  the  cord;  and 


ilar  to  3.  Neither  3,  4,  nor 
5  degenerate,  on  section  of 
the  cord,  from  which  it  is 
inferred    that    they     have 


THE  SPINAL   CORD.— GENERAL.  471 

they  may  also  connect  the  posterior  gray  cornua  with  the  cere- 
bellum as  6. 

Columns  of  Goll  (postero-median  columns). — They  do  not 
extend  beyond  the  lower  dorsal  or  upper  lumber  region ;  and 
their  fibers  pass  to  the  funiculi  graciles  of  the  medulla.  Ascend- 
ing degeneration  follows  section  of  these  columns. 

The  degenerations  referred  to  above  are  visible  by  the  micro- 
scope, and  of  the  character  following  section  of  nerves.  It  is 
probable  that  they  are  the  later  stages  of  a  primary  molecular 
derangement  in  consequence  of  interference  with  that  continu- 
ous functional  connection  between  all  parts  on  which  what  has 
been  called  nutrition,  but  which  we  have  shown  is  but  a  phase 
of  a  complex  metabolism,  depends. 

Decussation. — Sections  of  the  cord,  when  confined  to  one  lat- 
teral  half,  are  followed  by  paralysis  on  the  same  side  and  loss  of 
sensation,  confined  chiefly  to  the  opposite  half  of  the  body  be- 
low the  point  of  section.  The  results  of  experiment,  patho- 
logical investigation,  etc.,  have  rendered  it  clear  that — 1.  The 
great  majority  of  the  fibers  passing  between  the  periphery  and 
the  brain  decussate  somewhere  in  the  centers.  2.  Afferent  fibers 
cross  almost  directly  but  also  to  some  extent  along  the  whole 
length  of  the  cord  from  then'  point  of  entrance,  the  decussation 
being,  however,  completed  before  the  medulla  is  passed.  3. 
Motor  or  efferent  fibers  decussate  chiefly  in  the  medulla,  though 
crossing  is  continued  some  distance  down  the  cord,  such  latter 
fibers  being  but  a  small  portion  of  the  whole.  This  fact  is  best 
established,  perhaps,  by  noting  the  results  of  brain-lesions. 
With  few  exceptions,  susceptible  of  explanation,  a  lesion  of  one 
side  of  the  cerebrum  is  followed  by  loss  of  motion  of  the  oppo- 
site side  of  the  body.  These  are  all  central,  well-established 
truths.  It  is  also  now  pretty  well  determined  that  voluntary 
motor  impulses  descend  by  the  pyramidal  tracts,  both  the  direct 
and  the  crossed.  That  the  posterior  columns  of  the  cord  are  in 
some  way  concerned  with  sensory  impulses  there  is  no  doubt ; 
but  when  an  attempt  is  made  to  decide  details,  great  difficulties 
are  encountered.  Experiments  on  animals  are  of  necessity  very 
unsatisfactory  in  such  a  case,  from  the  difficulty  experienced  in 
ascertaining  their  sensations  at  any  time,  and  especially  when 
disordered. 

Pathological.  —  A  good  deal  of  stress  has  been  laid  upon 
the  teachings  of  locomotor  ataxia  in  the  human  subject.  The 
symptoms  of  this  disease  are  found  associated  with  lesions  of 


472 


COMPARATIVE   PHYSIOLOGY. 


the  posterior  columns  of  the  cord.  The  essential  feature  is  an 
inability  to  co-ordinate  movements,  though  muscular  power 
may  he  unimpaired.  But  such  inco-ordination  is  not  usually 
the  only  symptom ;  and,  while  the  disease  seems  usually  to 
begin  in  Burdach's  columns,  the  columns  of  Goll,  the  posterior 
nerve-roots,  and  even  the  cells  of  the  posterior  cornua,  may  be 
involved,  so  that  the  subject  becomes  very  complicated.  Co- 
ordination of  muscular  movements  is  normally  dependent  upon 
certain  afferent  sensory  impulses,  themselves  very  complex.  It 
is  to  be  remembered  also  that  there  are  numberless  connecting 
links  between  the  two  sides  of  the  cord  and  between  its  different 
columns  of  an  anatomical  kind,  not  to  mention  the  possibly 
numerous  physiological  (functional)  ones. 


Fig.  336. — Diagram  to  illustrate  probable  course  taken  by  fibers  of  nerve-roots  on  en- 
tering spinal  cord  (Schafer). 

We  have  stated  above  that  section  of  one  lateral  half  of  the 
cord  is  followed  by  loss  of  sensation  on  the  opposite  side  of  the 
body  ;  but  directly  the  contrary  has  been  maintained  by  other 
observers;  while  still  others  contend  that  the  effects  are  not 
confined  to  one  side,  though  most  pronounced  on  the  side  of 
the  section.     The  same  remark  applies  to  motion. 

While  there  is  considerable  agreement  as  to  the  pyramidal 
tracts  of  the  lateral  column,  the  functions  of  the  rest  of  these 


THE   SPINAL   CORD.— GENERAL. 


473 


divisions  of  the  cord  are  by  no  means  well  established.  It  is 
possible  that  vaso-motor,  respiratory,  and  probably  other  kinds 
of  impulses,  pass  by  portions  of  the  lateral  tracts  other  than 
the  crossed  pyramidal.  When  a  lateral  half  of  the  cord  is 
divided,  the  loss  of  function  is  not  permanent  in  all  instances, 
but  has  been  recovered  from  without  any  regeneration  of  the 
divided  fibers;  and  even  when  a  section  has  been  made  higher 
up  on  the  opposite  side,  partial  recovery  has  again  followed ; 
so  that  it  would  appear  that  impulses  had  pursued  a  zigzag 
course  in  such  cases.  We  do  not  think  that  such  experiments 
show  that  impulses  do  not  usually  follow  a  definite  course,  but 
that  the  resources  of  nature  are  great,  and  that,  when  one  tract 
is  not  available,  another  is  taken. 

It  is  plain  that  impulses  do  not  in  any  case  travel  by  one  and 
the  same  nerve-fiber  throughout  the  cord,  for  the  size  of  this 
organ  does  not  permit  of  such  a  view  being  entertained ;  at  the 
same  time  there  is  a  relation  between  the  size  of  a  cross-section 
of  the  cord  at  any  one  point  and  the  number  of  nerves  con- 
nected with  it  at  that  region. 

We  may  attempt  to  trace  the  paths  of  impulses  in  a  cord 
somewhat  as  follows:  1.  Volitional  impulses  decussate  chiefly 


c 

B 

A\bs 

1     V   IV    III    II    I      V     IV  III    II     I     XII  XI  X  IX  VIII VII  VI  V    IV   IU  II    I    VIII  VII  VI  V     IV   III  II     I 

Sacral.        Lumbar.  Dorsal.  Cervical. 

Fig.  337.— Diagram  to  illustrate  relative  and  absolute  extent  of  (1)  gray  matter,  (2) 
white  columns  in  successive  sectional  areas  of  spinal  cord,  and  (31  sectional  areas 
of  several  nerve-roots  entering  cord.  JVB,  nerve  roots;  AC,  LC,  PC,  anterior, 
lateral,  posterior  columns;  Gr,  gray  matter  (after  Schafer,  Ludwig,  and  Woro- 
schiloff). 


in  the  medulla  oblongata,  but  also,  to  some  extent,  throughout 
the  whole  length  of  the  spinal  cord.  They  travel  in  the  lateral 
columns  (crossed  pyramidal  tracts  chiefly,  if  not  exclusively), 
and  eventually  reach  the  anterior  roots  of  the  nerves  through 
the  anterior  gray  coruua,  passing  to  them,  possibly,  by  the  ante- 
rior columns.     From  the  cells  of  the  anterior  cornua,  impulses 


414 


COMPARATIVE  PHYSIOLOGY. 


travel  by  the  anterior  nerve-roots  to  the  motor  nerves,  by 
which  connection  is  made  with  the  muscles.  2.  Sensory  im- 
pulses enter  the  cord  from  the  afferent  nerve-fibers  by  the  pos- 


3' 4  25  6        6  524  3       V 


LOWER  LIMIT  OF 
MEDULLA 


4- 

5"/'        a 

Fig.  338.— Diagram  showing  course  of  fibers  in  spinal  cord  (after  Ranney).  1, 1',  direct 
pyramidal  bundles;  2,2',  crossed  pyramidal  bundles,  decussating  in  medulla;  3.3', 
direct  cerebellar  fibers;  4,4',  fibers  related  to  "muscular  sense,"  decussating  in 
medulla;  5,  5',  and  6,6',  fibers  relating  to  the  appreciation  of  touch,  pain,  and 
temperature.  The  motor  bundles  have  a  dot  upon  them  to  represent  the  motor 
cells  of  the  cord  (anterior  horn).  Note  that  the  motor  fibers  escape  from  the  ante- 
rior nerve-root  (a.  r.),  and  that  the  sensory  bundles  enter  at  the  posterior  nerve- 
root  (p.  v.),  which  has  a  ganglion  (rj)  upon  it. 

terior  nerve-roots,  passing  probably  by  the  posterior  columns  to 
the  posterior  cornua,  thence  to  the  lateral  columns,  decussation 
being  largely  immediate  though  not  completed  for  some  dis- 
tance up  the  cord. 

It  would  seem  that  the  lateral  columns  are  the  great  high- 


THE   SPINAL   CORD.— GENERAL.  475 

ways  of  impulses ;  though  in  all  instances  it  is  likely  that  the 
gray  matter  of  the  cord  plays  an  important  part  in  modify- 
ing them  before  they  reach  their  destination.  Some  observers 
believe  that  sensory  impulses  giving  rise  to  pain  travel  by  the 
gray  matter  of  the  cord  almost  exclusively.  It  would  be  easy 
to  lay  out  the  paths  of  impulses  in  a  more  definite  and  dog- 
matic manner  ;  but  the  evidence  does  not  seem  to  warrant  it, 
and  it  is  better  to  avoid  making  statements  that  may  require 
serious  modification,  to  say  the  least,  in  a  few  months.  The 
prominent  principle  to  bear  in  mind  seems  to  be  that  while 
there  are  tracts  in  the  cord  of  the  animals  that  have  been  exam- 
ined and  probably  of  all  that  have  well-formed  spinal  cords, 
along  which  impulses  travel  more  frequently  and  readily  than 
along  others,  it  is  equally  true  that  these  paths  are  not  invaria- 
ble, nor  are  they  precisely  the  same  for  all  groups  of  animals. 
The  cord  can  not  be  considered  independently  of  the  brain  ;  and 
there  can  be  no  doubt  that  the  paths  of  impulses  in  the  former 
are  related  to  the  constitution,  anatomical  and  physiological,  of 
the  latter.  It  is  still  a  matter  of  dispute  whether  the  cord  is 
itself  irritable  to  a  stimulus.  As  a  whole  it  is  without  doubt  ; 
as  also  the  white  matter  by  itself.  The  gray  matter  is  certainly 
conducting,  but  whether  irritable  or  not  is  still  doubtful.  Why 
the  sensibility  of  the  side  of  the  body  on  which  one  lateral  half 
of  the  cord  has  been  divided  should  be  increased  (hyperesthe- 
sia), is  also  undetermined.  Possibly  it  is  due  to  a  temporary 
disturbance  of  nutrition,  or  the  removal  of  certain  usual  inhibi- 
tory influences  from  above,  either  in  the  cord  or  brain. 

THE  AUTOMATIC  FUNCTIONS  OF  THE  SPINAL  CORD. 

Eeference  has  been  already  made  to  the  fact  that  when  por- 
tions of  a  mammaFs  cerebrum  are  removed  the  reflexes  of  the 
cord  become  more  pronounced,  owing  apparently  to  the  removal 
of  influences  operating  on  the  cord  from  higher  centers. 

When  the  cord  itself  is  completely  divided  across,  it  often 
happens  (in  the  dog,  for  example)  that  there  are  rhythmic 
movements  of  the  posterior  extremities — i.e.,  when  the  animal 
has  recovered  from  the  shock  of  the  operation — that  part  of  the 
cord  now  independent  of  the  rest  and  of  the  brain  seems  to 
manifest  an  unusual  automatism.  The  question,  however,  may 
be  raised  as  to  whether  this  is  a  purely  automatic  effect,  or  the 
result  of  reflex  action.      But,  whichever  view  be  entertained, 


476  COMPARATIVE  PHYSIOLOGY. 

these  phenomena  certainly  teach  the  dependence  of  one  part 
upon  another  in  the  normal  animal,  and  should  make  one  cau- 
tious in  drawing  conclusions  from  any  kind  of  experiment,  in 
regard  to  the  normal  functions.  As  we  have  often  urged  in 
the  foregoing  chapters,  what  a  part  may  under  certain  circum- 
stances manifest,  and  what  its  behavior  may  he  as  usually 
placed  in  its  proper  relations  in  the  body,  are  entirely  different, 
or  at  least  may  be.  When  one  leg  is  laid  over  the  other  and  a 
sharp  blow  struck  upon  the  patella  tendon,  the  leg  is  jerked  up 
in  obedience  to  muscular  contraction.  It  is  not  a  little  difficult 
to  determine  whether  this  result  is  due  to  direct  stimulation  of 
the  muscle  or  to  reflex  action,  the  first  link  in  the  chain  of 
events  necessary  to  call  it  forth  originating  in  the  tendon  ; 
hence  the  term  tendon-reflex.  But  at  present  it  is  safer  to 
speak  of  it  as  the  u knee-jerk,"  or  the  "tendon-phenomenon." 
It  disappears,  however,  when  the  spinal  cord  is  destroyed  or  is 
diseased,  as  in  locomotor  ataxia,  or  when  the  nerves  of.  the 
muscles  or  the  posterior  nerve-roots  are  divided,  showing  that 
the  integrity  of  the  center,  the  nerves,  and  the  muscles  are  all 
essential.  There  are  normally  many  such  phenomena  (reflexes) 
besides  the  "knee-jerk." 

Another  question  very  difficult  to  decide  is  that  relating  to 
the  usual  condition  of  the  muscles  of  the  living  animal.  It  is 
generally  admitted  that  the  muscles  of  the  body  are  all  in  a 
somewhat  stretched  condition,  but  it  is  not  so  clear  whether 
the  skeletal  muscles  are  under  a  constant  tonic  influence  like 
those  of  the  blood-vessels.  It  is  certain  that,  when  the  nerves 
going  to  a  set  of  muscles  are  cut,  when  even  the  posterior  roots 
of  the  nerves  related  to  the  part  involved  are  divided  or  the 
spinal  cord  destroyed,  there  is  an  unusual  flaccidity  of  the 
limb  involved.  But  the  natural  condition  may  be,  it  has  been 
suggested,  the  result  of  reflex  action.  The  subject  is  probably 
more  complex  than  it  has  hitherto  been  considered. 

The  facts  of  such  a  case— those  of  the  tendon-phenomenon 
and  similar  ones — would  be  better  understood  if  the  spinal 
cord,  the  nerves,  and  the  muscles  associated  with  them,  were 
regarded  as  parts  of  a  whole  so  connected  in  their  functions 
that  severance  of  any  one  of  them  leads  to  disorder  of  the  rest. 
That  the  cells  of  the  cord  are  constantly  exercising  an  influence 
through  the  nerves  on  the  muscles,  while  they  in  turn  do  not 
lead  an  independent  existence,  but  are  as  constantly  influenced 
by  afferent  impulses,  and  that  one  of  the  results  is  the  condi- 


THE  SPINAL   CORD.— GENERAL.  477 

tion  of  the  muscles  referred  to,  is,  we  are  convinced,  the  case. 
To  say  that  it  is  either  entirely  automatic  or  purely  reflex,  or 
that  the  whole  of  the  facts  would  be  covered  even  by  any  com- 
bination of  these  two  processes,  would  probably  be  unjustifiable. 
The  influence  of  the  centers  over  the  metabolism  of  parts  is 
both  constant  and  essential  to  their  well-being  ;  and  in  such  a 
case  as  that  now  considered  it  may  be  that  a  certain  degree  of 
tonus  is  normal  to  a  healthy  muscle  in  its  natural  surround- 
ings in  the  body. 

There  is  now  considerable  evidence  in  favor  of  placing  cer- 
tain centers  presiding  over  the  lower  functions,  as  micturition, 
defecation,  erection  of  penis,  etc.,  in  the  spinal  cord  of  mam- 
mals, especially  its  lower  part — which  centei^s,  if  they  be  not 
automatic,  are  not  reflex  in  the  usual  sense ;  but  their  considera- 
tion is  better  attempted  in  connection  with  the  treatment  of  the 
physiology  of  the  parts  over  which  they  preside. 

SPECIAL   CONSIDERATIONS. 

Comparative. — Among  invertebrates  there  is,  of  course,  no 
spinal  cord,  but  each  segment  of  the  animal  is  enervated  by  a 
special  ganglion  (or  ganglia)  with  associated  nerves.  Neverthe- 
less, these  are  all  so  connected  that  there  is  a  co-ordination, 
though  not  so  pronounced  as  in  the  vertebrate,  in  which  the 
actual  structural  bonds  are  infinitely  more  numerous,  and  the 
functional  ones  still  more  so.  From  this  result  possibilities  to 
the  vertebrate  unknown  to  lower  forms  ;  at  the  same  time,  in- 
dependent life  and  action  of  parts  are  necessarily  much  greater 
among  invertebrates,  as  evidenced  especially  by  the  renewal  of 
the  whole  animal  from  a  single  segment  in  many  groups,  as  in 
certain  divisions  of  worms,  etc. 

It  also  follows  from  the  same  facts  that  a  vertebrated  ani- 
mal must  suffer  far  more  from  injury,  in  consequence  of  this 
greater  dependence  of  one  part  on  another  ;  a  thousand  things 
may  disturb  that  balance  on  which  its  well-being,  indeed,  its 
very  life  hangs.  It  is  noticeable,  moreover,  that,  as  animals 
occupy  a  higher  place  in  the  organic  scale,  their  nervous  sys- 
tem becomes  more  concentrated  ;  ganglia  seem  to  have  been 
fused  together,  and  that  extreme  massing  seen  in  the  spinal 
cord  and  brain  of  vertebrates  is  foreshadowed.  In  the  chapters 
on  the  brain  numerous  illustrations  of  the  nervous  system  in 
lower  forms  will  be  found. 


478  COMPARATIVE   PHYSIOLOGY. 

The  fact  that  the  brain  and  cord  arise  from  the  same  germ 
layer,  and  up  to  a  certain  point  are  developed  almost  precisely- 
alike,  is  full  of  significance  for  physiology  as  well  as  morphol- 
ogy. That  original  deep-lying  connection  is  never  lost,  though 
functional  differentiation  keeps  pace  with  later  morphological 
differentiation.  But  even  among  vertebrates  the  spinal  cord 
shows  a  complexity  gradually  increasing  with  ascent  in  the 
organic  series.  In  the  lowest  of  the  fishes  or  vertebrates  (Am- 
phioxus  lanceolatus)  the  creature  possesses  a  spinal  cord  only 
and  no  brain,  so  that  an  opportunity  is  afforded  of  witness- 
ing how  an  animal  deports  itself  in  the  absence  of  those  direct- 
ive functions,  dependent  on  the  existence  of  higher  cerebral 
centers.  The  Lancelet  spends  a  great  part  of  its  life  buried  in 
mud  or  sand  on  the  bottom  of  the  ocean,  and  its  existence  is 
very  similar  to  that  of  an  invertebrate,  though,  of  course,  the 
dependence  of  parts  on  each  other  is  somewhat  greater. 

Evolution. — According  to  the  general  law  of  habit  and  in- 
heritance, we  should  suppose  that  at  birth  each  group  of  ani- 
mals would  manifest  those  reflex  and  other  functions  of  the 
cord  which  were  peculiar  to  its  ancestors.  Observation  and 
experiment  both  show  that  reflexes,  etc.,  are  hereditary  ;  that 
they  tend  to  become  more  and  more  so  with  each  generation  ; 
and  at  the  same  time  that  habit  or  exercise  is  essential  for  their 
perfect  development.  They  stand,  in  fact,  in  the  same  relation 
as  instincts,  which  are  closely  connected  with  them.  Like  the 
latter,  they  may  be  modified  by  way  of  increase  or  diminution 
and  otherwise.  To  illustrate,  it  can  not  be  doubted  that  gallop- 
ing is  the  natural  gait  of  horses,  as  shown  by  the  tendency  of 
even  good  trotters  to  "break  "  or  pass  into  a  gallop  ;  but  it  is 
equally  well  known  that  famous  trotters  breed  trotters.  In 
other  words,  an  acquired  gait  becomes  organized  in  the  nervous 
system  (especially)  of  the  animal,  and  is  transmitted  with  more 
and  more  fixity  and  certainty  with  the  lapse  of  time.  But  all 
experience  goes  to  show  that  walking,  running,  or  any  of  the 
movements  of  animals  are,  when  fully  formed  as  habit-reflexes, 
dependent  for  their  initiation  on  the  will  in  most  but  not  all 
instances,  and  require  for  their  execution  certain  combinations 
of  sensory  and  other  afferent  impulses,  and  the  integrity  of  a 
vast  complex  of  nervous  connections  in  the  spinal  cord. 

It  is  well  known  that  one  in  a  period  of  absent-mindedness 
will  walk  into  a  building  to  which  he  was  accustomed  to  go 
years  before,  though  not  of  late,  showing  plainly  that  volition 


THE   SPINAL  CORD.— GENERAL.  479 

was  not  momentarily  required  for  the  act  of  walking  and  all  else 
that  is  involved  in  the  above  behavior.  It  suggests  that  certain 
nervous  and  muscular  connections  have  been  formed,  function- 
ally at  least.  Plainly,  then,  we  should  not  expect  each  indi- 
vidual man's  spinal  cord  to  be  the  same,  but  that  the  series  of 
mechanisms  of  which  every  spinal  cord  is  made  up  should  differ 
with  experience  ;  and  if  this  holds  for  individuals,  how  much 
more  must  it  be  true  of  different  groups  of  animals,  the  habits 
of  which  differ  so  widely. 

All  the  facts  go  to  show  that  the  cord  is  made  up  of  nervous 
mechanisms— if  we  may  so  speak — which  are  naturally  associ- 
ated, both  structurally  and  functionally,  with  certain  nerves 
and  muscles ;  these,  like  the  paths  which  impulses  take  to  and 
from  the  brain,  though  usual,  are  not  absolutely  fixed,  though 
more  so  as  reflex  than  conducting  paths,  while  they  are  con- 
stantly liable  to  be  modified  in  action  by  the  condition  of 
neighboring  groups  of  mechanisms,  etc. 

We  have  said  less  about  the  gray  matter  of  the  cord  as  a 
conductor  than  its  importance  perhaps  deserves.  It  is  believed 
by  many  that  impulses  which  give  rise  to  sensations  of  pain 
always  travel  by  the  gray  matter ;  and  there  is  not  a  little  evi- 
dence to  show  that,  when  none  of  the  white  columns  are  avail- 
able, owing  to  operative  procedure,  disease,  or  other  disabling 
cause,  the  gray  matter  will  conduct  impulses  that  usually  pro- 
ceed by  other  tracts. 

Synoptical. — The  spinal  cord  is  composed  of  large  ganglionic 
nerve-cells,  fibers,  and  connecting  neuroglia.  Functionally  it 
is  a  conductor,  the  seat  of  certain  automatic  centers  and  of 
reflex  mechanisms.  Probably  in  every  case  the  one  function  is 
to  a  certain  extent  associated  with  the  other — i.  e.,  when  the 
cord  acts  reflex!  y  it  is  also  a  conductor,  and  the  cells  concerned 
are  so  readily  excited  to  certain  discharges  of  nervous  energy 
that  automaticity  is  suggested,  and  so  in  other  instances :  thus, 
in  the  case  of  automaticity,  reflex  influence  or  afferent  impulses 
are  with  difficulty  entirely  excluded  from  consideration. 

The  great  majority  of  conducting  fibers  seem  to  cross  either 
in  the  cord  itself  or  in  the  medulla  oblongata.  The  conducting 
paths  that  have  been  shown  by  pathological  and  clinical  inves- 
tigation to  be  best  marked  out  in  the  spinal  cord  are  those  for 
voluntary  motor  impulses.  So  far  as  the  functions  of  the 
human  organ  are  concerned,  clinical  and  pathological  facts 
have  thrown  the  greatest  amount  of  direct  light  on  the  subject; 


480  COMPARATIVE   PHYSIOLOGY. 

but  the  inferences  thus  drawn  have  been  modified  and  supple- 
mented by  the  results  of  experiments  on  certain  otber  mam- 
mals. 

It  is  especially  important  to  bear  in  mind  that,  while  certain 
conducting  paths  are  usual,  they  are  not  invariable:  in  like 
manner,  reflex  impulses  may  not  be  confined  to  usual  groups  of 
cells,  but  may  extend  widely,  and  so  briug  into  action  a  large 
number  of  muscles.  The  resulting  reflex  in  any  case  is  depend- 
ent on  the  character,  intensity,  and  location  of  the  stimulus, 
and  especially  on  the  condition  of  the  central  cells  involved. 
In  the  whole  functional  life  of  the  cord  the  influence  of  higher 
centers  in  the  organ  itself  and  especially  in  tbe  brain  is  to  be 
considered.  The  cord  is  rather  a  group  of  organs  than  a 
single  one. 


THE   BKAIN. 


At  the  outset  we  may  remark  that  the  whole  subject  will 
be  studied  more  profitably  if  it  be  borne  in  mind  that — 1.  The 
brain  is  rather  a  collection  of  organs,  bound  together  by  the 
closest  anatomical  and  physiological  ties  than  a  single  one ;  in 
consequence  of  which  it  is  quite  impossible  to  understand  the 
normal  function  of  one  part  without  constantly  bearing  in 
mind  this  relationship.  This  aspect  of  the  subject  has  not  re- 
ceived the  attention  it  deserves.  No  one  regards  the  aliment- 
ary tract  as  a  single  organ ;  but  it  is  likely  that  the  dependence 
functionally  of  one  part  of  the  digestive  canal  upon  another 
is  not  more  intimate  than  that  established  in  that  great  collec- 
tion of  organs  crowded  together  and  making  up  the  brain.  2. 
Since  the  relative  size,  position,  and  anatomical  connections  of 
the  parts  that  make  up  the  brain  are  different  in  different 
groups  of  animals,  not  to  speak  of  the  fact  that  the  functions 
of  any  part  of  the  brain  of  an  animal,  like  that  of  its  spinal 
cord,  already  alluded  to,  must  depend  in  great  part  upon  its 
own  and  its  inherited  ancestral  experiences,  it  follows  that  the 
greatest  caution  must  be  exercised  in  applying  conclusions  true 
of  one  group  of  animals  to  another.  3.  It  follows  from  what 
has  been  referred  to  in  1  above,  that  conclusions  based  upon  the 
behavior  of  an  animal  after  section  or  removal  of  a  part  of  the 
brain  must  be,  until  at  least  corrected  by  other  facts,  received 
with  some  hesitation.  4.  It  also  might  be  inferred  from  1  that 
it  is  desirable  to  study  the  simpler  forms  of  brain  found  in  the 
lower  vertebrates,  in  order  to  prepaid  for  the  more  elaborate 
development  of  the  encephalon  in  the  higher  mammals  and  in 
man.  5.  The  embryological  development  of  the  organ  also 
thi'ows  much  light  upon  the  whole  subject. 

The  student  will  see  from  these  remarks  that  a  sound  knowl- 
edge of  the  anatomy  of  the  brain  and  its  connections  is  indis- 
pensable for  a  just  appreciation  of  its  physiology;  nor  must 
31 


482  COMPARATIVE   PHYSIOLOGY. 

such  knowledge  be  confined  to  any  single  form  of  the  organ. 
There  is  only  one  way  by  which  this  can  be  attained :  dissection, 
with  the  help  of  plates  and  descriptions.  The  latter  alone  fre- 
quently impart  ideas  that  are  quite  erroneous,  though  they 
serve  an  especially  good  purpose  in  helping  to  fix  the  pictures 
of  the  natural  objects,  and  in  reviving  them  when  they  have 
become  dim. 

It  is  neither  difficult  to  obtain  nor  to  dissect  the  brain  of  the 
fish,  frog,  bird,  etc.  Valuable  material  may  be  saved  and  the 
subject  approached  profitably,  if,  prior  to  the  dissection  of  a 
human  brain,  a  few  specimens  from  some  group  or  groups  of  the 
domestic  animals  be  examined.  However  useful  artificial  brain 
preparations  may  be,  they  are  so  far  from  nature  in  color,  con- 
sistence, and  many  other  properties,  that,  taken  alone,  they  cer- 
tainly may  serve  greatly  to  mislead ;  and  we  hope  the  student 
will  allow  us  to  urge  upon  him  the  methods  above  suggested 
for  getting  real  lasting  knowledge.  The  figures  given  below 
may  prove  helpful  when  supplemented  as  we  advise. 

The  great  difference  in  total  size,  and  in  the  relative  propor- 
tion, situation,  etc.,  of  parts,  will,  however,  be  obvious,  from  the 
figures  themselves ;  and  as  we  have  already  pointed  out  more 
than  once,  the  preponderance  of  the  cerebrum  in  man  must 
ever  be  borne  in  mind  in  the  consideration  of  his  entire  organi- 
zation,- whether  physical,  mental,  or  moral. 

ANIMALS   DEPRIVED   OF   THE    CEREBRUM. 

The  cerebrum  may  be  readily  removed  from  a  frog,  without 
producing  either  severe  prolonged  shock  or  any  considerable 
haemorrhage.  Such  an  animal  remains  motionless,  unless 
wben  stimulated,  though  in  a  somewhat  different  position  from 
that  of  a  frog,  having  only  its  spinal  cord.  It  can,  however, 
crawl,  leap,  swim,  balance  itself  on  an  inclined  plane,  and  when 
leaping  avoid  obstacles.  One  looking  at  such  an  animal  per- 
forming these  various  acts  would  scarcely  suspect  that  any- 
thing was  the  matter  with  it,  so  perfectly  executed  are  its  move- 
ments. We  are  forced  to  conclude,  from  its  remaining  quiet, 
except  when  aroused  by  a  stimulus,  that  its  volition  is  lost;  but, 
apart  from  that,  and  the  fact  that  it  evidently  does  not  see  as 
well  as  before,  it  appears  to  be  normal.  It  has  no  intelligent 
directive  power  over  its  movements.  It  remains,  therefore,  to 
explain  how  it  is  that  they  are  so   much  more  complete,  so 


THE   BRAIN.  483 

much  better  co-ordinated  in  the  entire  animal  than  when  only 
the  spinal  cord  is  left.  It  seems  to  be  legitimate  to  infer  that 
the  other  parts  of  the  brain  contain  the  nervous  machinery  for 
this  work,  which  is  usually  aroused  to  action  by  the  will,  but 
which  an  external  stimulus  may  render  active.  All  the  connec- 
tions, structural  and  functional,  are  present,  except  those  on 
which  successful  volition  depends.  The  frog  with  the  cord 
only,  sinks  at  once  when  thrown  into  water;  when  gently 
placed  on  its  back,  it  may  and  probably  will  remain  in  that 
position,  without  an  attempt  at  recovery.  There  is,  in  fact, 
very  limited  power  of  co-ordination. 

Removal  of  the  cerebral  lobes  in  the  bird  is  more  likely  to 
be  attended  with  difficulties,  and  conclusions  must  be  drawn 
with  greater  caution. 

But  a  pigeon  may  be  kept  alive  after  such  an  operation  for 
months.  It  can  stand,  balancing  on  one  leg ;  recover  its  posi- 
tion when  placed  on  its  side;  fly  when  thrown  into  the  air; 
it  will  even  preen  its  feathers,  pick  up  food,  and  drink  water. 
Its  movements  are  such  as  we  might  expect  from  a  stupid,  drow- 
sy, or  probably  intoxicated  bird ;  but  it  is  plainly  endowed  with 
vision,  though  not  as  good  as  before.  But  spontaneous  move- 
ments are  absent,  and  the  pecking  at  food,  etc.,  must  be  consid- 
ered as  associate  reflexes,  and  as  such  are  very  interesting,  in 
that  they  show  Iloav  machine-like,  after  all,  many  of  the  appar- 
ently volitional  acts  of  animals  really  are.  In  a  mammal  so 
great  is  the  shock,  etc.,  resulting  from  the  operative  procedure, 
that  the  actual  functions  of  the  remaining  parts  of  the  brain, 
when  the  cerebral  convolutions  are  removed,  are  greatly  ob- 
scured ;  nevertheless,  little  doubt  is  left  on  the  mind  that  homol- 
ogous parts  discharge  analogous  functions.  It  can  walk,  run, 
leap,  right  itself  when  placed  in  an  unnatural  position,  eat  when 
food  is  placed  in  its  mouth,  and  avoid  obstacles  in  its  path, 
though  not  perfectly.  Yet  it  remains  motionless  unless  stimu- 
lated ;  all  objects  before  its  eyes  impress  it  alike  if  at  all.  The 
animal  evidently  has  neither  volition  nor  intelligence.  Now,  if 
any  of  the  parts  between  the  cerebrum  and  the  medulla  be 
removed  the  creature  shows  lessened  co-ordinating  power;  so 
that  the  inference  that  these  various  parts  are  essential  constitu- 
ents of  a  complex  mechanism,  all  the  components  of  which 
are  necessary  to  the  highest  forms  of  muscular  co-ordination 
and  probably  other  functions,  is  unavoidable. 

Since  we  are  dealing  with  co-ordinated  movements,  we  may 


484  COMPARATIVE  PHYSIOLOGY. 

now  treat  of  the  functions  of  a  portion  of  the  ear,  according  to 
our  present  classification. 


HAVE   THE    SEMICIRCULAR   CANALS  A  CO-ORDINAT- 
ING FUNCTION? 

Physiologists  have  as  yet  been  unable  to  assign  to  the  semi- 
circular canals  a  function  in  hearing,  and  upon  certain  results, 
partly  of  disease  but  chiefly  of  experiment,  it  has  been  con- 
cluded, though  somewhat  dubiously,  that  they  are  concerned 
with  those  sensations  that  conduce  to  or  are  essential  to  main- 
tenance of  the  sense  of  equilibrium  ;  in  a  word,  that  they  are 
the  organs  of  that  sense  in  the  same  way  that  the  eye  is  the 
organ  of  vision. 

Until  further  evidence  is  forthcoming,  we  are  not  inclined 
to  give  assent  to  the  existence  of  any  mechanism  in  the  semi- 
circular canals,  affording  sensory  data  so  entirely  different 
from  those  furnished  by  other  recognized  (and  unrecognized) 
sense-organs,  that  upon  them  alone,  or  in  a  manner  entirely 
their  own,  arises  a  consciousness  of  equilibrium.  We  are  in- 
clined to  regard  the  latter  as  depending  upon  the  fusion  in  con- 
sciousness of  a  vast  complex  of  sensations  ;  and  that  upon  the 
whole  being  there  represented,  or  a  portion  wanting,  depends 
either  the  preservation  of  equilibrium,  or  a  partial  or  entire  loss 
of  the  same.  Nevertheless,  it  is  highly  probable  that  sensory 
impulses  of  a  very  important  character,  in  addition  to  such  as 
are  essential  for  hearing,  may  proceed  from  the  semicircular 
canals,  and  indeed  other  parts  of  the  labyrinth  of  the  ear. 

FORCED  MOVEMENTS. 

When  certain  portions  of  the  brain  of  the  mammal  have 
been  injured,  movements  of  a  special  character  result,  and,  inas- 
much as  they  are  not  voluntary,  in  the  ordinary  sense  at  least, 
have  been  spoken  of  as  forced  or  compulsory.  The  movements 
may  be  classified  according  as  they  are  around  the  long,  the 
vertical  or  the  transverse  axis  of  the  body  of  the  animal.  Hence 
there  are  "  circus  "  movements,  when  the  creature  simply  turns 
about  in  a  circle,  "  rolling  "  movements,  etc.  These  and  others 
may  be  toward  or  from  the  side  of  injury.  While  in  some 
cases  there  may  be  a  certain  amount  of  muscular  weakness  in 
consequence  of  the  injury,  which  may,  in  part,  account  for  the 


THE  BRAIN.  485 

direction  of  the  movements,  this  is  not  so  in  all  cases ;  nor  does 
it,  in  itself,  explain  the  fact  of  their  being  plainly  not  volun- 
tary in  the  usual  sense. 

The  parts  of  the  brain,  which,  when  injured,  are  most  liable 
to  be  followed  by  forced  movements  are  the  basal  ganglia  (cor- 
pora striata  and  optic  thalami),  the  crura  cerebri,  corpora  quad- 
rigemina,  pons  Varolii,  and  medulla  oblongata,  and  especially 
if  the  section  be  unilateral.  We  have  already  seen  that  several 
of  these  parts  are  concerned  in  muscular  co-ordination  ;  hence 
the  disorderly  character  of  any  movements  that  might  now  re- 
sult when  any  part  of  this  related  mechanism  is  thrown  out  of 
gear,  so  to  speak  ;  but,  apart  from  that,  we  think  that  the  view 
presented  in  the  previous  sections  is  applicable  in  this  case  also, 
while  the  forced  movements  themselves  throw  light  upon  the 
symptoms  following  injury  to  the  semicircular  canals.  When 
that  constant  afflux  of  sensory  impulses  toward  the  nervous 
centers  is  interfered  with,  as  must  be  the  case  in  such  sections 
as  are  now  referred  to,  it  is  plain  that  the  balance  in  conscious- 
ness must  be  disturbed  ;  confusion  results,  and  it  is  not  sur- 
prising that,  instead  of  a  passive  condition,  one  marked  by  dis- 
orderly movements  should  result  in  an  animal,  since  movement 
so  largely  enters  into  its  life-habits.  It  is  important  to  remem- 
ber, in  this  connection,  that  the  great  highway  of  impulses 
between  tbe  cerebral  cortex  and  other  parts  of  the  brain  and 
the  spinal  cord  lies  in  the  very  parts  of  the  encephalon  we  are 
now  considering. 


FUNCTIONS  OP  THE  CEREBRAL  CONVOLUTIONS. 

Comparative.— It  will  conduce  to  the  comprehension  of  this 
subject  if  some  reference  be  now  made  to  the  development  of 
the  brain  in  the  different  groups  of  the  animal  kingdom. 

Invertebi-ates  not  only  have  no  cerebrum,  but  no  brain  in 
the  strict  sense  of  the  term  as  applied  to  the  higher  mammals. 
In  most  forms  of  this  great  subdivision  of  the  animal  kingdom, 
the  first  or  head  segment  is  provided  with  ganglia  arranged  in 
the  form  of  a  collar  around  the  oesophagus,  by  means  of  com- 
missural nerve  connections  ;  so  that  the  nervous  supply  of  the 
head  is  not  widely  different  from  that  of  the  other  segments 
of  the  body.  But  as  we  ascend  in  the  scale  among  the  in- 
vertebrates these  ganglia  become  more  crowded  together,  and 
so  resemble  the  vertebrate  brain  with  its  massed  ganglia  and 


486 


COMPARATIVE   PHYSIOLOGY. 


numerous  connections  through  nerve-fibers,  etc.  But  in  this 
respect  we  find  great  difference  among  vertebrates.  We  can 
recognize,  on  passing  upward  from  the  Amphioxus,  destitute 
of  a  brain  proper,  to  man,  all  gradations  in  the  form,  relative 
size,  multiplicity  of  connecting  ties,  etc. 

Speaking  generally,  there  is  great  difference  in  the  weight 
of  the  cerebrum,  both  relative  and  absolute.  In  all  animals  be- 
low the  primates  (man  and  the  apes)  the  cerebellum  is  either 
not  at  all  or  but  imperfectly  covered  by  the  cerebrum  ;  wbile 


Fig.  339. — Nervous  system  of  medicinal  leech  (after  Owen),  a,  double  supra-oesopha- 
geal  ganglion  connected  with  rudimentary  ocelli  (b,  b)  by  nerves;  c,  double  infra- 
cesophageal  ganglionic  mass,  which  is.  continuous  with  double  ventral  cord,  hav- 
ing compound  ganglia  at  regular  intervals. 

in  man,  so  great  is  the  relative  size  of  the  latter,  that  the 
cerebellum  is  scarcely  visible  from  above.  If  we  except  the 
elephant,  in  which  the  brain  may  reach  the  weight  of  ten 
pounds,  and  the  whale  with  its  brain  of  more  than  five  pounds 
-a, 
V 


Fig  340.— Brain  and  cranial  nerves  of  perch,  seen  from  the  side  (after  Gegenbaur  and 
Cuvier).  A,  cerebral  lobe  with  olfactory  ganglion  in  front;  7?,  optic  lobe;  C,  cerc- 
■  bellum;  />,  medulla  oblongata;  I—VIIT,  nerves  in  usual  order;  K,  lateral  branch 
of  vagns;  I,  upper  twig  of  same;  m,  dorsal  branch  of  trigeminus,  joined  by  n,  dor- 
sal branch  of  vagus;  o,  /3,  y,  three  branches  of  trigeminus;  Se,  facial  nerve;  \, 
branchial  branches  of  vagus. 


THE   BRAIN. 


487 


in  the  largest  specimens,  the  brain  of  man  is  even  absolutely 
heavier  than  that  of  any  other  animal,  which  is  in  great  part 
due  to  the  preponderating  development  of  the  cerebrum. 

While  the  cerebral  surface  is  smooth  in  all  the  lower  verte- 
brates, and  but  little  convoluted  until  the  higher  mammals  are 
reached,  the  brain  of  the  primates,  and  especially  of  man.  has 
its  surface  enormously  increased,  owing  to  its  numerous  fis- 
sures and  convolutions,  which,  in  fact,  arise  from  the  growth 


Fig.  341. — Brain  and  spinal  cord  of  frog  (Bastian).  A,  olfactory  lobes;  B,  cerebral 
lobes;  R,  pineal  body;  C,  D,  optic  lobes;  E,  cerebellum;  H,  spinal  cord.  The 
cerebellum  is  notably  small. 

of  the  organ  being  out  of  proportion  to  that  of  the  bony  case 
in  which  it  is  contained  ;  and  since  those  cells  which  go  to 
make  up  the  gray  matter  and  are  devoted  to  the  highest  func- 
tions, are  disposed  over  the  surface,  the  importance  of  the  fact 
in  accounting  for  the  superior  intelligence   of  the   primates, 


Fig.  343. 


Fh.o.-? 


Fig.  343. 


Fig.  342.— Brain  of  the  pike,  viewed  from  above  (Huxley).  ^4,  the  olfactory  nerves  or 
lobes,  and  beneath  them  the  optic  nerves;  B,  the  cerebral  hemispheres;  C,  the 
optic  lobes;  T).  the  cerebellum. 

Fig.  343.— The  t>rain  of  edible  frog  (Eana  esculenta).  1x4.  (After  Huxley.)  L.ol, 
the  rhinencephalon,  or  olfactory  lobes,  with  7,  the  olfactory  nerves;  He.  the  cere- 
bral hemispheres;  Fh.o,  the  thalamencephalon  with  the  pineal  gland,  P/r,  L.op, 
optic  lobes;  0,  cerebellum;  8.  rh,  the  fourth  ventricle;  Mo,  medulla  oblongata. 


488 


COMPARATIVE  PHYSIOLOGY. 


A 


Fig.  344. — A,  C,  the  brain  of  a  lizard  (Psammosaurus  Bengalensis),  and  B,  D,  of  a 
bird  (Meleagris  gallopavo,  the  turkey),  drawn  as  if  they  were  of  equal  lengths 
(after  Huxley).  A.  B,  viewed  from  above;  C,  D,  from  the  left  side.  Olf,  olfactory 
lobes;  Pn,  pineal  gland;  Hmp,  cerebral  hemispheres;  Mb,  optic  lobes  of  the  mid- 
brain; Cb,  cerebellum;  M.  O,  medulla  oblongata;  ii,  iv,  vi,  second  fourth,  and 
sixth  pairs  of  cerebral  nerves;  Py,  pituitary  body. 


Fri;.  8 1").  —Brains  of  a  lizard  (Psammosaurns  Benr/alensifs)  and  of  a  bird  {Meleagris 
gallopava)  in  longitudinal  and  vertical  section.    The  upper  figure  represents  the 


THE   BRAIN. 


489 


lizard's  brain;  the  lower,  that  of  the  bird  (after  Huxley  and  Carus).  Letters  as  in 
the  preceding  figure,  except  L.  t,  lamina  terminaUs,  or  anterior  wall  of  the  third 
ventricle;  /.  .)/,  foramen  of  Munro;  a,  anterior  commissure;  Th.  E,  thalamen- 
cephalon;  s,  soft  commissure;  p,  posterior  commissure;  iv,  indicates  the  exact 
point  of  exit  of  the  fourth  pair  from  that  part  of  the  brain  which  answers  to  the 
value  of  Vieussens. 

and  especially  of  man,  becomes  apparent.  Depth  of  Assuring 
is,  however,  of  more  importance  than  multiplicity  of  furrows  ; 
and  it  may  be  observed  that  intelligence  is  not  always  in  pro- 
portion to  the  extent  to  which  the  cerebral  surface  is  broken 
up  into  fissures  and  convolutions.  Tbe  depth  of  the  gray  mat- 
ter is  also  very  variable,  and  seems  to  bear  an  important  rela- 
tion to  psychic  development.  Man's  brain,  then,  is  character- 
ized by  its  great  size  and  complexity  ;  while  those  parts  treated 
elsewhere,  concerned  in  co-ordination,  vision,  etc.,  are  well 
developed,  the  cerebrum,  especially  its  convolutions  as  distin- 
guished from  its  basal  ganglia,  is,  out  of  all  proportion,  greater 
than  in  any  other  animal. 

The  gray  matter  of  the  brains  of  the  higher  vertebrates  is 
distributed  as  masses  of  ganglionic  cells  internally,  and  as  a 
fairly  uniform  layer  over  its  surface.  The  brain  of  man  weighs 
about  three  pounds  on  the  average,  that  of  the  male  being 
a  few  ounces  (four  to  six)  heavier  than  that  of  the  female. 


Fig.  346. 


Fig.  347. 


Fig.  348. 


Fig.  346.— Brain  of  pigeon  (after  Ferrierl.    A,  cerebral  hemispheres;  B,  optic  lobe;  C, 

cerebellum,  the  lateral  lobes  of  which  are  very  small. 
Fig.  347. — Brain  and  spinal  cord  of  chick  at  sixteen  days  old;  optic  lobes,  b,  are  still 

in  contact  (after  Owen  and  Anderson). 
Fig.  348.— Brain  and  part  of  spinal  cord  of  chick  twenty  days  old,  showing  optic  lobes 

widely  separated  and  cerebellum,  c,  largely  developed. 


The  individual  and  race  differences,  though  considerable,  are 
not  comparable  ha  degree  to  those  that  distinguish  man  from 
even  the  highest  apes,  the  brain  of  the  latter  weighing  not 
more  than  about  one  third  as  much  as  that  of  the  human  sub- 
ject. While  it  has  been  shown  that  individual  men  and  women, 
having  brains  of  average  or  even  sub-medium  weight,  may  reach 


490 


COMPARATIVE   PHYSIOLOGY. 


even  distinction  in  the  intellectual  world;  and  though  idiots 
have  been  known   to  possess   brains  abnormally  heavy,  it  is 


Pig.  350. 

Fig.  349. — Outer  surface  of  brain  of  horse  (after  Solly  and  Leuret).    e,  olfactory  lobe; 

h,  hippocampal  lobe  (processus  pyriformis);  1,2,3,  lobes  of  cerebellum;  o,  optic 

nerve;  m,  motor  oculi;  p,  fourth  nerve;  t,  fifth  nerve;  u,  sixth  nerve;  /,  facial; 

I,  auditory;  r/,  glossopharyngeal;  v,  vagus;  s,  spinal  accessory;   n,  hypoglossal; 

X,  pons  Varolii. 
Pig.  350. — Longitudinal  section  through  center  of  brain  of  horse,  presenting  view  of 

internal  surface  (after  Solly  and  Leuret).    c.c,  corpus  callosum;  p,  thalamus;  co, 

middle  commissure;  t.  q,  corpora  quadrigemina,  in  front  of  which  is  the  pineal 

body.    The  cerebellum  lias  been  cut  through. 

nevertheless  true  that  brain-weight  and  the  higher  powers  of 
man  bear  a  close  though  not  invariable  relationship.  The 
apparent  discrepancies  are  susceptible  of  explanation. 

Besides  the  gray  matter,  with  its  cells  of  highest  functional 
value  from  the  standpoint  now  taken,  the  brain  consists,  and 
in  large  part,  of  neuroglia  and  nerve-fibers,  with  probably 
chiefly,  and  in  the  case  of  the  fibers  solely,  a  conducting  func- 
tion. 


THE  BRAIN. 


491 


The  Connection  of  one  Part  of  the  Brain  with  another  - 
Though  it  has  long  been  known  that  the  different  parts  of  the 


suDra-orbit-il-   7  p    \r  j?   t  tr  ,,  „    ■  eyjviannssure,  /».  the  insula;  S.  Or, 

S.  Oc,  M.  Oc,  I.  Oc,  the  three  occipUal'gyri.  '  '       ^  ^  three  temPoraI>  and 


492 


COMPARATIVE  PHYSIOLOGY. 


brain  were  connected  by  bridges  of  fibers  (commissures,  etc.), 
tbe  physiological  significance  of  the  fact  seems  to  have  been 
largely  ignored,  and  even  at  the  present  day  is  too  little  con- 


PlG.  352.— Inner  views  of  cerebral  hemispheres  of  the  rabbit,  pig,  and  chimpanzee, 
drawn  as  before,  and  placed  in  the  same  order  (Huxley).  01,  olfactory  lobe;  C.c, 
corpus  callosum  ;  A. C,  anterior  commissure;  H,  hippocampal  sulcus;  Vh,  unci- 
nate; M,  marginal;  6',  callosal  gyri;  /.  P,  internal  perpendicular;  C'a,  calcarine; 
Coll,  collateral  sulci;  /'',  fornix. 

sidered.     1.   Cerebral  fibers  pass  between  the  convolutions  of 
this  part  of  the  brain  and  the  cerebellum ;  between  the  former 


t-pr=  "Y^y  \"<^f 


494 


COMPARATIVE   PHYSIOLOGY. 


mr.     J 


Fig.  354.— Diagrammatic  horizontal  section  of  a  vertebrate  brain  (Huxley).  The  follow- 
ing letters  serve  for  both  this  figure  and  the  one  following.  Mb,  mid-brain.  What 
lies  in  front  of  this  is  the  fore-brain,  and  what  lies  behind,  the  hind-brain.  L.  t, 
the  lamina  terminalis;  Olf,  olfactory  lobes;  limp,  hemispheres;  T/i.  E,  thala- 
mencephalon;  Pn,  pineal  gland;  Py,  pituitary  body;  FM,  foramen  of  Munro;  CS, 
corpus  striatum;  Th,  optic  thalamus;  CQ,  corpora  quadrigemina;  CC,  crura  cere- 
bri; Cb,  cerebellum;  PV,  pons  Varolii;  MO,  medulla  oblongata;  /,  olf ac tori  i;  II, 
optici;  III,  point  of  exit  from  brain  of  motores  oculorum;  IV,  of  pathetici;  VI, 
of  abducentes;  V—XII,  origins  of  the  other  cerebral  nerves.  1,  olfactory  ven- 
tricle; 2,  lateral  ventricle;  3,  third  ventricle;  4,  fourth  ventricle;  +,  iter  a  tertio 
ad  quartum  ventriculum. 

and  the  main  basal  ganglia;  between  the  gray  matter  of  the 
convolutions  on  the  same  side,  and  between  the  latter  and  those 


Fio.  355.— A  longitudinal  and  vertical  section  of  a  vertebrate  brain  (Huxley).  Letters 
as  above.  The  lamina  terminalis  is  represented  by  the  strong  black  line  between 
FMimA  Z. 


THE  BRAIN.  495 

on  the  opposite  halves;  between  the  gray  matter  of  the  cortex 
and  the  internal  capsule,  the  corpora  striata,  optic  thalami,  pons 
Varolii,  the  medulla  oblongata,  and  so  to  the  spinal  cord.  The 
course  of  the  latter  tracts  of  fibers  have  been,  especially  by  the 
help  of  pathology,  definitely  followed.  Some  of  these  connec- 
tions are  given  in  more  detail  below. 

1.  Cerebro-cerebellar  fibers,  (a.)  From  the  cortical  cells  of 
the  anterior  cerebral  lobe  to  tbe  pons  Varolii,  passing  through 
the  internal"  capsule  and  thence  through  the  lower  and  outer 
part  of  the  crus  cerebri  (crusta).  (b.)  Fibers  from  the  occipital 
and  temporo-sphenoidal  lobes,  passing  by  the  crusta,  reach  the 
upper  surface  of  the  cerebellum. 

2.  Fibers  bridging  the  tivo  sides  of  the  cerebrum,  (a.)  By 
means  of  the  corpus  callosum  chiefly,  passing  from  the  gray 
matter  in  the  first  instance.  (6.)  From  the  temporo-sphenoidal 
lobe  on  each  side  through  the  corpora  striata  and  anterior  com- 
missure, (c.)  Fibers  from  the  upper  part  of  the  crus  cerebri 
(tegmentum)  to  the  optic  thalamus  of  each  side  and  onward 
to  the  temporo-sphenoidal  lobes,  forming  the  posterior  com- 
missure. 

3.  Fibers  connecting  different  parts  of  the  cerebral  convolu- 
tions on  the  same  side.  These  are  exceedingly  numerous  and 
belong  to  such  tracts  as  the  "arcuate  fibers,"  passing  from  one 
gyrus  to  another;  "collateral  fibers,"  forming  distant  convo- 
lutions; fibers  of  the  fornix  between  the  uncinate  gyrus,  hip- 
pocampus major,  and  optic  thalamus;  longitudinal  fibers  of  the 
corpus  callosum;  fibers  of  the  taenia  semicircularis,  uncinate 
fasciculus,  etc. 

4.  Fibers  forming  the  cerebrum  and  the  spinal  cord.  Ac- 
cording as  they  pass  downward  or  upward  do  they  converge  or 
diverge,  and  the  most  important  seem  to  pass  through  the  in- 
ternal capsule ;  and  while  the  majority  do  perhaps  form  some 
connection  either  with  the  corpora  striata  and  optic  thalami, 
some  seem  to  pass  directly  downward  through  the  internal  cap- 
sule. It  is  held  by  many  that  the  fibers  passing  through  the 
posterior  portion  of  the  internal  capsule  are  derived  from  the 
posterior  lobe  of  the  cerebrum,  and  are  the  paths  of  sensory  im- 
pulses upward ;  while  the  rest  of  the  internal  capsule  is  made 
up  of  fibers  from  the  anterior,  and  especially  the  middle  portion 
of  the  cerebral  cortex  (motor  area),  and  these  fibers  are  the 
paths  of  motor  (efferent)  impulses. 

It  now  becomes  clearer  that  the  brain  is  constituted  a  whole 


496  COMPARATIVE   PHYSIOLOGY. 

by  such  connections ;  and  that,  apart  from  the  multiplicity  of 
cells  with  different  functions  to  perform,  situated  in  different 


Fig.  356.— Diagrammatic  representation  of  the  course  of  some  of  the  fibers  in  the  cere- 
brum of  man  (after  Le  Bon). 

areas,  the  complexity  and  at  the  same  time  the  unity  of  the 
encephalon  becomes  increasingly  evident,  merely  upon  anatomi- 
cal grounds;  but  we  shall  find  such  a  view  still  further  strength- 
ened by  study  of  the  functions  of  the  various  parts.  While  the 
tracts  enumerated  are  anatomical  and  have  been  clearly  traced, 
there  can  be  little  doubt  that  many  others  yet  remain  to  be 
marked  out ;  and  that,  apart  from  such  collections  of  fibers,  we 
must  recognize  functional  paths  by  the  neuroglia,  and  possibly 
others  still.  It  is  not  to  be  forgotten  that  in  the  brain,  as  in  the 
spinal  cord,  nerve-cells  are  themselves  conductors,  and  while 


THE   BRAIX. 


497 


there  may  be  certain  areas  within  which  the  resistance  is  such 
that  impulses  are  usually  confined  to  them,  it  is  also  true  that, 
as  in  the  cord,  there  may  be  a  kind  of  overflow.  Adjacent  cells, 
possibly  widely  separated  cells,  may  become  involved.  We  shall 
return  to  tbis  important  subject  again,  however,  as,  without 
recognizing  such  relationships,  it  seems  to  us  quite  impossible 
to  understand  the  facts  as  we  find  them  in  the  working  of  the 
body  and  the  mind. 


The  Cerebral  Cortex. — We  may  now  proceed  to  inquire  what 
are  the  functions  of  the  cells  of  the  gray  matter  covering  the 
surface  of  the  cerebrum.     Before  the  birth  of  physiology  as  a 

32 


49S 


COMPARATIVE   PHYSIOLOGY. 


science,  Gall  recognized  and  taught  that  the  encephalon  is  a  col- 
lection of  organs ;  that  these  have  separate  functions ;  that  the 
relative  size  of  each  determines  the  degree  of  its  functional  ac- 
tivity ;  and  that  the  cranium  developing  in  proportion  to  the 
growth  of  the  brain,  the  former  might  give  information  as  to 
the  probable  size  of  what  lay  beneath  it  in  different  regions. 
It  will  be  seen  that,  as  thus  interpreted,  phrenology  is  a  very 
different  thing  from  what  usually  passes  under  that  name,  and 
is  paraded  before  wondering  audiences  by  ignorant  charlatans. 
In  the  main  the  doctrines  of  Gall  are  not  without  a  certain 
foundation  in  facts ;  and  the  modern  theory  of  localization  of 
function  bears  some  resemblance  to  what  Gall  taught,  though 
with  greater  limitations. 


FlG.  358.—  Outer  surface  of  cerebrum  (after  Exner).  The  shaded  portion  represents 
the  motor  area  in  man  and  the  monkey— i.  e.,  the  area  which  most  observers  be- 
lieve to  be  associated  with  certain  voluntary  movements  of  the  limbs,  etc. 


In  the  mean  time  it  has  been  found  that  in  many  cases  it 
was  possible  to  locate  the  site  of  a  brain-lesion  (tumor,  etc.)  by 
the  symptoms,  chiefly  motor,  of  the  patient ;  and  brain-surgery 


THE  BRAIN.  499 

has  in  consequence  entered  upon  a  new  era  of  development. 
Tumors  thus  localized  have  been  removed  successfully,  and  the 
patients  restored  to  health.  As  a  result  of  the  various  kinds  of 
observations  and  discussions  on  this  subject  of  late  years,  the 
localizationists  are  willing  to  admit  that  the  areas  of  the  cortex 
can  not  be  marked  off  mathematically — that,  in  fact,  they 
"overlap."  This  is  in  itself  an  important  concession.  Again, 
there  is  less  confidence  in  the  location  of  the  various  sensory 
centers  than  of  the  motor  centers.  Most  investigators  are  be- 
lievers in  a  "  motor  area  "  par  excellence  (for  the  arm,  leg,  etc.) 
around  the  fissure  of  Eolando  (Fig.  358).  This  view  is  now,  so 
far  as  man  is  concerned,  widely  accepted. 

There  is  agreement  in  placing  the  sensory  centers  behind 
the  above-mentioned  motor  area,  and  especially  in  the  occipital 
lobes.  The  tendency  to  locate  a  visual  center  in  this  region  is 
growing  stronger.  There  is  much  disagreement  as  to  the  other 
sensory  centers  formerly  placed  in  the  angular  gyrus  and  tem- 
poro-sphenoidal  lobes.  The  intellectual  faculties  have  not  been 
located  in  any  such  sense  as  Gall  and  his  followers  attempted 
to  establish.  The  first  two  frontal  convolutions  are  those,  per- 
haps, to  which  localization  has  as  yet  been  least  applied. 
Chiefly  on  clinical  and  pathological  grounds  a  center  for 
speech  has  long  been  located  in  the  third  (left)  frontal  convolu- 
tion (Broca's)  and  parts  immediately  behind  it.  It  has  been  ob- 
served that  when  disease  attacks  this  area  speech  is  interfered 
with  in  some  way. 

We  may  say  then,  generally,  that  the  tendency  at  the  pres- 
ent time,  both  on  the  part  of  physiologists  and  clinical  ob- 
servers, is  to  admit  localization  to  some  degree  and  in  some 
sense.  This  has  been  the  result  in  part  of  experiments  on  the 
dog  and  especially  on  the  monkey,  combined  with  the  discus- 
sion of  clinical  cases  which  resulted  in  death  (followed  by  an 
autopsy),  or  of  others  marked  by  a  successful  diagnosis  and  re- 
moval of  lesions  or  other  treatment.  In  other  words,  the  truth, 
if  it  is  to  be  reached  at  all,  must  be  sought  by  the  plan  we 
have  advocated  throughout  this  work — the  discussion  of  the  re- 
sult's of  as  inany  different  methods  as  can  be  brought  to  bear  on 
this  or  any  other  subject.  Neither  the  experimental  nor  the 
pathological  method  alone  can  settle  such  complex  questions. 
Although  localization  of  function  has  not  been  established  for 
the  cerebral  cortex  in  the  case  of  those  animals  with  which  the 
practitioner  of  veterinary  medicine  has  to  deal  as  it  has  for  man 


500  COMPARATIVE   PHYSIOLOGY. 

and  the  monkey,  we  have  thought  it  well  to  bring  the  subject 
before  the  student  of  comparative  medicine,  since  it  can  not  be 
doubted  that  future  research  will  put  the  physiology  of  the 
brains  of  the  domesticated  animals  in  a  new  light,  in  doing  which 
guidance  will  naturally  be  sought  from  what  has  been  already 
clone,  more  especially  in  the  case  of  the  human  subject  and  his 
nearest  allies.  Some  would  maintain  that  in  the  case  of  the 
dog,  motor  and  sensory  localization  has  been  established ;  that 
in  this  animal  there  is  a  motor  area  in  the  region  of  the  crucial 
sulcus  corresponding  to  that  around  the  fissure  of  Rolando  in 
man.  The  subject  is,  however,  far  from  finally  settled  even  in 
the  case  of  the  dog,  the  brain  of  which  has  been  more  thoroughly 
investigated  than  that  of  any  other  of  our  domestic  animals. 
Very  little  can  as  yet  be  said  in  regard  to  cortical  localization 
in  the  horse,  ox,  etc.  It  seems  highly  probable  that  investiga- 
tion will  show  that  cortical  localization  in  the  primates  (man 
and  the  monkey  tribe)  exists  in  a  far  higher  degree  than  in 
any  other  animals. 

The  Circulation  in  the  Brain. — The  brain,  being  inclosed 
within  an  air-tight  bony  case,  its  circulation  is  of  necessity 
peculiar.  Since  any  undue  compression  of  the  encephalon  may 
lead  to  even  a  fatal  stupor,  it  is  clear  that  there  must  exist  some 
provision  to  permit  of  the  excess  of  arterial  blood  that  is  re- 
quired for  unusual  activity  of  the  brain.  It  is  to  be  borne  in 
mind  that  the  fluid  within  the  ventricles  is  continuous,  through 
the  foramen  of  Magendie  in  the  roof  of  the  fourth  ventricle, 
with  that  surrounding  the  spinal  cord  (spinal  cavity) ;  so  that 
an  increase  in  the  volume  of  the  encephalon  in  consequence  of 
an  afflux  of  blood  might  be  in  some  degree  compensated  by  an 
efflux  of  the  cerebro-spinal  fluid.  The  part  played  by  this  ar- 
rangement has,  however,  been  probably  overestimated.  But 
the  peculiar  venous  sinuses  do,  it  is  likely,  serve  to  regulate  the 
blood-supply ;  being  very  large,  they  may  answer  as  temporary 
overflow  receptacles.  An  inspection  of  the  fontanelles  of  an 
infant  reveals  a  beating  corresponding  with  the  pulse;  and, 
when  a  large  part  of  the  cranium  is  removed  in  an  animal,  a 
plethysmograph  shows  a  rise  in  volume  corresponding  with 
the  pulse  and  the  respiratory  movements,  as  in  the  case  of  the 
fontanelles.  But,  besides  these,  periodic  waves  of  contraction 
are  now  known  to  pass  over  the  cerebral  arteries. 

Whether  the  latter  is  part  of  a  general  wave  traversing  the 
whole  arterial  system  is  as  yet  uncertain.     Though  there  is 


THE  BRAIN.  501 

considerable  anastomosis  of  vessels  in  the  encephalon,  it  is  not 
equal  to  what  takes  place  in  many  other  organs.  It  is  well 
known  that  a  clot  or  other  plug  within  a  cerebral  vessel  is  more 
serious  than  in  many  other  regions,  which  is  partly  to  be  ex- 
plained by  the  lack  of  sufficient  anastomosis  for  the  vascular 
needs  of  the  parts.  It  is  also  well  known  that,  in  organs  which 
constitute  parts  of  a  related  series,  as  the  different  divisions  of 
the  alimentary  tract,  all  are  not  usually  at  the  same  time  vas^ 
cular  to  the  same  extent.  While  they  act  functionally  in  rela- 
tion to  each  other,  they  exemplify  also  a  certain  degree  of  inde- 
pendence. Such  a  condition  of  things  is  now  known  to  exist  in 
the  brain — i.  e.,  certain  areas  may  be  abundantly  supplied  with 
blood  as  compared  with  others :  and  it  seems  highly  probable 
that  a  condition  of  equal  arterial  tension  throughout  is  scarcely 
a  normal  condition.  Though  the  quantity  of  blood  contained 
within  the  vessels  of  the  whole  brain  at  any  one  time  is  not  so 
large  as  in  some  other  organs  (glands),  yet  the  foregoing  facts 
and  the  rapidity  of  the  flow  must  be  taken  into  account.  The 
capillaries  are  very  close  and  abundant,  in  the  gray  matter  es- 
pecially ;  and  it  is  to  be  borne  in  mind  that  it  is  chiefly  these 
vessels  which  are  concerned  in  the  actual  metabolism  (nutri- 
tion) of  parts.  However,  the  chemical  changes  in  the  nervous 
system  being  feeble,  it  would  appear  probable  that  it  does  its 
work  with  less  consumption  of  pabulum  than  other  parts  of 
the  body.  We  wish  to  lay  stress  on  the  local  nature  of  vascular 
dilatation  in  the  brain,  as  it  greatly  assists  in  explaining  certain 
phenomena  about  to  be  considered. 

Sleep. — Observations  upon  animals  from  which  portions  of 
the  cranium  have  been  removed,  so  that  the  brain  was  visible, 
show  that  during  sleep  the  blood-vessels  are  much  less  promi- 
nent than  usual ;  and  it  is  well  known  that  means  calculated  to 
diminish  the  circulation  in  the  brain,  as  cold  and  pressure,  favor 
sleep.  It  is  also  well  established  by  general  experience  that 
withdrawal  of  the  usual  afferent  impulses  through  the  various 
senses  favors  sleep.  A  remarkable  case  is  on  record  of  a  youth 
whose  avenues  for  sensory  impressions  were  limited  to  one  eye 
and  a  single  ear,  and  who  could  be  sent  to  sleep  by  closing 
these  against  the  outer  world.  Yet  this  subject  after  a  long 
sleep  would  awake  of  his  own  accord,  showing  that,  while  affer- 
ent impulses  have  undoubtedly  much  to  do  with  maintaining 
the  activity  of  the  cerebral  centers,  yet  their  automaticity  (in- 
dependence) must  also  be  recognized. 


502  COMPARATIVE   PHYSIOLOGY. 

It  is  a  matter  of  common  experience  that  weariness,  or  the 
exhaustion  following  on  pain,  mental  anxiety,  etc.,  is  favorable 
to  sleep. 

A  good  deal  of  light  is  thrown  on  this  subject  by  hiberna- 
tion, particularly  in  mammals. 

From  special  study  of  the  subject  we  have  ourselves  learned 
that,  however  temperature,  and  certain  other  conditions  may 
influence  this  state,  it  will  appear  at  definite  periods  in  defiance, 
to  a  large  extent,  of  the  conditions  prevailing.  Hibernation, 
we  are  convinced,  is  marked  by  a  general  slowing  of  all  of  the 
vital  processes  in  which  the  nervous  system  takes  a  prominent 
part.  Sleep  and  hibernation  are  closely  related.  In  both  there 
is  a  diminution  of  the  rate  of  the  vital  processes,  as  shown  by 
the  income  and  output,  measured  by  chemical  standards,  with 
of  course  obvious  physical  signs,  as  slowed  respiration,  circula- 
tion, etc.  While  sleep,  then,  is  primarily  the  result  of  a  rhyth- 
mical retardation  of  the  vital  processes,  especially  within  the 
nervous  system,  it  is  like  hibernation  in  some  degree  (in  the 
lowest  creatures,  without  a  nerve  system)  the  outcome  of  that 
rhythm  impressed  on  every  cell  of  the  organism  and  the  influ- 
ence of  which  is  felt  in  a  thousand  ways,  that  no  doubt  we  are 
quite  unable  to  recognize. 

Hypnotism. — By  the  help  of  the  above  principles  the  sub- 
ject of  hypnotism,  now  of  absorbing  interest,  may  be  in  great 
part  explained.  This  condition  is  characterized  by  loss  of  vo- 
lition and  judgment.  It  may  be  induced  in  man  and  certain 
other  animals  by  prolonged  staring  at  a  bright  object,  assisted 
by  a  concentration  of  the  attention  on  that  alone,  as  far  as  pos- 
sible, combined  with  a  condition  of  mental  passivity  in  other 
respects.  The  individual  gradually  becomes  drowsy,  and  finally 
falls  into  a  state  in  many  respects  strongly  resembling  sleep. 

Hypnotism  proper  may  be  combined  with  catalepsy,  a  con- 
dition in  which  the  limbs  remain  rigid  in  whatever  condition 
tbey  may  be  placed.  Modifications  of  the  vascular  and  respira- 
tory systems  occur.  Various  animals  have  been  hypnotized,  as 
the  fowl,  rabbit,  Guinea-pig,  crayfish,  frog,  etc.  This  condi- 
tion is  readily  induced  in  the  common  fowl,  more  especially 
tbe  wilder  individuals,  by  holding  the  creature  with  the  bill 
down  on  a  table  and  the  whole  animal  perfectly  quiet  for  a 
short  time.  Upon  the  removal  of  the  pressure  the  bird  re- 
mains perfectly  passive  and  apparently  asleep  for  some  little 
time. 


THE   BRAIN, 


503 


Fig.  359.— Lateral  surface  of  brain  of  monkey,  displaying  motor  areas  (after  Horsley 
and  Schafer). 


Fk..  860.-  Median  surface  of  brain  of  monkey  (after  Horsley  and  Schafer). 
Figs.  359  and  300  may  be  said  to  embody  the  views  of  Horsley  and  Schafer  more  espe- 
cially in  regard  to  motor  localization. 


50A 


COMPARATIVE   PHYSIOLOGY. 


FUNCTIONS   OF  OTHER   PORTIONS   OF   THE  BRAIN. 

Certain  parts  of  the  encephalon  are  spoken  of  as  the  basal 
ganglia,  prominent  among  which  are  the  corpus  striatum  and 
the  optic  thalamus. 

The  Corpus  Striatum  and  the  Optic  Thalamus.— The  corpus 
striatum  consists  of  several  parts,  the  main  divisions  being  an 
intra- ventricular  portion  or  caudate  nucleus,  and  an  extra-ven- 
tricular part  or  lenticular  nucleus. 


Fio.  361. — Transverse  section  of  cerebral  hemispheres  of  man  at  level  of  cerebral  gan- 
glia (after  Dalton).  1,  great  longitudinal  fissure;  2,  part  of  same  between  occipital 
lobes;  3,  anterior  part  of  corpus  callosum;  4,  fissure  of  Sylvius;  5,  convolutions 
of  island  of  Rei)  (insula);  6,  caudate  nucleus  of  corpus  striatum;  7,  lenticular  nu- 
cleus of  corpus  striatum;  8,  optic  thalamus;  9,  internal  capsule;  10,  external  cap- 
sule; 11,  claustrum. 


THE   BRAIN. 


505 


Between  these  lies  the  internal  capsule,  through  which 
pass  fibers  that  spread  out  toward  the  cortex,  as  the  corona 
radiata. 

Pathology,  especially,  has  shown  that  a  lesion  of  the  intra- 
ventricular portion  of  the  corpus  striatum,  and,  above  all,  of 
the  internal  capsule,  is  followed  by  failure  of  voluntary  move- 
ment (akinesia).  It  would  appear  that  a  great  part  of  the 
fibers  from  the  motor  area  around  the  fissure  of  Rolando,  pass 
through  the  intra-ventricular  parts  of  the  corpus  striatum,  and 
especially  its  internal  capsule.  But  it  is  also  to  be  borne  in 
mind  that  a  large  part  of  the  fibers  passing  from  the  cortex 
make  connection  with  the  cells  of  the  corpus  striatum  before 
reaching  the  cord.  These  facts  render  the  occurrence  of  loss  of 
voluntary  motor  power  comprehensible. 

The  fibers  of  the  peduncles  of  the  brain  may  be  divided  into 
an  interior  or    lower   division  (cmista),  going  mostly  to  the 


Fig.  362. — Transverse  section  of  human  brain  (after  Dalton).  This  and  the  preceding 
figure  are  somewhat  diagrammatic.  1,  pons  Varolii;  -2,2.  crura  cerebri;  3,3,  in- 
ternal capsule;  4,  4,  corona  radiata;  5,  optic  thalamus;  (j,  lenticular  nucleus;  7. 
corpus  callosum. 


corpus  striatum,  and  a  posterior  division  (tegmentum),  passing 
principally  to  the  optic  thalami ;  many,  possibly  most  of  them, 
ultimately  reach  the  cortex.  Many  clinical  observers  do  not 
hesitate  to  speak  of  the  optic  thalamus  as  sensory  in  function. 


506  COMPARATIVE  PHYSIOLOGY. 

and  the  corpus  striatum,  as  inotor ;  but  the  clinical  and  patho- 
logical evidence  is  conflicting — all  lesions  of  these  parts  not 
being  followed  by  loss  of  sensation  and  motion  respectively ; 
though  an  injury  to  the  internal  capsule  generally  results  in 
paralysis.  All  are  agreed  that  the  symptoms  are  manifested  on 
the  side  of  the  body  opposite  to  the  side  of  the  lesion,  so  that  a 
decussation  must  take  place  somewhere  between  the  ganglion 
and  the  periphery  of  the  body. 

There  is  no  doubt  that  the  optic  thalamus,  especially  its 
posterior  part,  is  concerned  with  vision,  for  injury  to  it  is  fol- 
lowed by  a  greater  or  less  degree  of  disturbance  of  this  func- 
tion. As  has  been  already  pointed  out,  unilateral  injury  of 
either  of  these  ganglia  leads  to  inco-ordination  or  to  forced 
movements.  That  these  regions  act  some  intermediate  part  in 
the  transmission  of  impulses  to  and  from  the  brain  cortex,  and 
that  the  anterior  one  is  concerned  with  motor,  and  the  pos- 
terior possibly  with  sensory  (tactile,  etc.),  and  certainly  with 
visual  impulses,  may  be  stated  with  some  confidence,  though 
further  details  are  not  yet  a  subject  of  general  agreement. 

Corpora  Quadrigemina. — The  function  of  these  parts  in  vis- 
ion, as  in  the  co-ordination  of  the  movements  of  the  ocular 
muscles,  and  their  relations  to  the  movements  of  the  pupil,  will 
be  considered  later.  However,  the  actual  centers  for  these  func- 
tions seem  to  lie  in  the  anterior  portion  of  the  floor  of  the 
aqueduct  of  Sylvius,  and  are  indirectly  affected  by  stimulation 
of  the  corpora  quadrigemina.  Extirpation  of  these  parts  on 
one  side  produces  blindness  of  the  opposite  eye,  and  in  birds, 
etc.,  the  same  result  follows  when  their  homologues — the  optic 
lobes — are  similarily  treated.  There  can  be  no  doubt,  therefore, 
that  they  are  a  part  of  the  central  nervous  machinery  of  vision, 
and  it  seems  to  be  probable  that  the  anterior  parts  of  the  cor- 
pora quadrigemina  alone  have  this  visual  function.  But,  since 
it  is  the  opposite  eye  that  is  affected,  and  in  some  animals 
(rabbits)  that  alone,  we  are  led  to  infer  a  decussation  of  the 
optic  fibers,  or  at  least  of  impulses.  In  dogs,  on  the  other  hand, 
the  crossing  seems  to  be  but  partial. 

It  begins  to  appear  that  there  are  several  parts  of  the  brain 
concerned  with  vision.  After  removal  of  almost  any  part  of 
the  cerebral  cortex,  if  of  sufficient  extent,  vision  is  impaired. 
We  may  say,  then,  that  before  an  object  is  "  seen  "  in  the  high- 
est sense,  processes  beginning  in  the  retina  undergo  further 
elaboration  in  the  corpora  quadrigemina,  optic  thalami,  and, 


THE   BRAIN. 


507 


finally  in  the  cerebral  cortex.  We  may  safely  assume  that  the 
part  played  by  the  latter  is  of  very  great  importance,  making 
the  perception  assume  that  highest  completeness  which  is  of 


Pig.  303. — Diagrammatic  representation  of  brain  on  transverse  section  to  illustrate 
course  of  fibers  (after  Landois).  C,  C,  cortex  cerebri;  C.s,  corpus  striatum:  X.I, 
lenticular  nucleus;  T.o,  optic  thalamus;  P.  peduncle;  H,  tegmentum;  j>.  crusta; 
V,  corpora  quadrigemina;  1. 1,  corona  radiata  of  corpus  striatum;  2,  2,  of  1cm  icu- 
lar  nucleus;  3,  8,  of  optic  thalamus;  4,  4,  of  corpora  quadrigemina;  5,  direct  fibers 
to  cortex  cerebri  (Fleehsig);  6,  6,  fibers  from  corpora  quadrigemina  to  tegmentum; 
m,  further  course  of  these  fillers;  S.  s.  tillers  from  corpus  striatum  and  lenticular 
nucleus  to  crusta  of  peduncle  of  cerebrum;  M,  further  course  of  these;  ■'\  3,  course 
of  sensory  fibers;  Ii,  transverse  section  of  spinal  cord;  v.  W,  anterior,  and  h.  H". 
posterior  roots;  a,  a,  system  of  association  fibers;  c.  c,  commissural  fibers. 


508  COMPARATIVE   PHYSIOLOGY. 

very  varying  character,  no  doubt,  with  different  groups  of  ani- 
mals. In  a  sense,  all  mammals  may  see  alike,  and,  in  another 
sense,  they  may  see  things  very  differently ;  for,  if  we  may  judge 
by  the  differences  in  this  respect  between  educated  and  unedu- 
cated men,  the  great  dissimilarity  lies  in  the  interpretation  of 
what  is  seen ;  in  a  word,  the  cortex  has  to  do  with  the  perfect- 
ing of  visual  impulses.  Nevertheless,  a  break  anywhere  in  the 
long  and  complicated  chain  of  processes  must  lead  to  some 
serious  impairment  of  vision.  Much  of  the  same  sort  of  reason- 
ing applies  to  the  other  senses  and  also  to  speech. 

To  speak,  therefore,  of  a  visual  center  or  a  speech  center  in 
any  very  restricted  sense  is  unjustifiable ;  at  the  same  time,  it 
is  becoming  clearer  that  there  is  in  the  occipital  lobe,  rather 
than  in  other  parts  of  the  cortex,  an  area  which  takes  a  pecul- 
iar and  special  share  in  elaborating  visual  impulses  into  visual 
sensations  and  perceptions  ;  and  there  can  be  little  doubt 
that  the  other  senses  are  represented  similiarly  in  the  cerebral 
cortex. 

The  Cerebellum. — Both  physiological  and  pathological  re- 
search point  to  the  conclusion  that  the  cerebellum  has  an  im- 
portant share  in  the  co-ordination  of  muscular  movements. 
Ablation  of  parts  of  the  organ  leads  to  disordered  movements  ; 
and,  when  the  whole  is  removed  in  the  bird,  co-ordination  is 
all  but  impossible,  and  the  same  holds  for  mammals.  Section 
of  the  middle  peduncle  of  one  side  is  liable  to  give  rise  to  roll- 
ing forced  movements.  In  fact,  injury  to  the  cerebellum  causes 
symptoms  very  similar  to  those  following  section  of  the  semi- 
circular canals,  so  that  many  have  thought  that  in  the  latter 
case  the  cerebellum  had  itself  been  injured. 

Pathological. — Tumors  and  other  lesions  frequently,  though 
not  invariably,  give  rise  to  unsteadiness  of  gait,  much  like  that 
affecting  an  intoxicated  person.  It  may  safely  be  said  that  the 
cerebellum  takes  a  very  prominent  share  in  the  work  of  the 
muscular  co-ordination  of  the  body. 

As  has  already  been  pointed  out,  several  tracts  of  the  spinal 
cord  make  connection  with  the  cerebellum,  and  it  is  not  to  be 
forgotten  that  this  part  of  the  brain  has,  in  general,  most  ex- 
tensive connections  with  other  regions.  Insufficient  study  has 
as  yet  been  given  to  the  cerebellum,  and  it  is  likely  that  the 
part  it  takes  in  the  functions  of  the  encephalon  is  greater  than 
has  yet  been  rendered  clear.  The  old  notion  that  this  organ 
bears  any  direct  relation  to  the  sexual  functions  seems  to  be 


THE   BRAIN.  509 

without  foundation.  It  has  now  been  clearly  demonstrated 
that  the  lower  region  of  the  spinal  cord  is,  in  the  dog  and  prob- 
ably most  mammals,  the  part  of  the  nerve-centers  essential  for 
the  sexual  processes. 

Crura  Cerebri  and  Pons  Varolii.— As  has  been  already  noted, 
the  peduncles  (crura)  are  the  paths  of  impulses  from  certain 
parts  of  the  cerebral  cortex,  the  basal  ganglia,  and  the  spinal 
cord.  The  functions  of  the  gray  matter  of  the  crura  are  un- 
known. But,  since  forced  movements  ensue  on  unilateral  sec- 
tion, it  is  plain  that  they  also  have  to  do  with  muscular  co- 
ordination. 

The  transverse  fibers  of  the  pons  Varolii  connect  the  two 
halves  of  the  cerebellum.  Its  longitudinal  fibers  have  extensive 
connections— the  anterior  pyramids  and  olivary  bodies  of  the 
medulla,  the  lateral,  and  perhaps  also  a  part  of  the  posterior 
columns  of  the  cord,  while  upward  these  fibers  connect  with 
the  crura  cerebri  and  so  with  the  cortex. 

Pathological. — Paralysis  of  the  face  usually  occurs  on  the 
same  side  as  that  of  the  rest  of  the  body  ;  hence  it  must  be 
inferred  that  there  is  a  decussation  somewhere  of  the  fibers  of 
the  facial  nerve  ;  but  there  is  much  still  to  be  learned  about 
this  subject. 

Medulla  Oblongata. — In  some  animals  (frogs)  it  is  certainly 
known  that  this  region  of  the  brain  has  a  co-ordinating  func- 
tion, and  it  is  probable  that  it  is  concerned  with  such  uses  in 
all  animals  that  possess  the  organ,  or  rather  collection  of  organs, 
seeing  that  this  part  of  the  brain  must  be  regarded  as  especially 
a  mass  of  centers,  the  functions  of  which  have  been  already 
considered  at  length.  So  long  as  the  medulla  is  intact,  life  may 
continue  ;  but,  except  under  special  circumstances,  which  do 
not  invalidate  this  general  statement,  its  destruction  is  followed 
by  the  death  of  the  animal. 

We  may  simply  enumerate  the  centers  that  are  usually 
located  in  the  medulla  :  The  respiratory  (and  convulsive),  car- 
dio-inhibitory,  vaso-motor,  center  for  deglutition,  center  for 
the  movements  of  the  gullet,  stomach,  etc.,  and  the  vomiting 
center  ;  center  for  the  seci'etion  of  saliva  and  possibly  other  of 
the  digestive  fluids.     Some  add  a  diabetic  and  other  centers. 


510 


COMPARATIVE   PHYSIOLOGY. 


SPECIAL   CONSIDERATIONS, 

Embryological. — The  further  we  progress  in  the  study  of  the 
nervous  system,  the  greater  the  significance  of  the  facts  of  its 


Fig.  364.— Vertical  longitudinal  section  of  brain  of  human  embryo  of  fourteen  weeks. 
1x3.  (After  Sharpey  and  Reichert.)  c,  cerebral  hemisphere;  cc,  corpus  callosum 
beginning  to  pass  back;  /,  foramen  of  Munro;  p,  membrane  over  third  ventricle 
and  the  pineal  body;  th,  thalamus;  3,  third  ventricle;  /.  olfactory  bulb ;  cq,  corpora 
quadrigemina;  cr,  crura  cerebri,  and  above  them,  aqueduct  of  Sylvius,  still  wide; 
c',  cerebellum,  and  below  it  the  fourth  ventricle;  jw,  pons  Varolii;  m,  medulla 
oblongata. 

early  development  becomes.     It  will  be  remembered  that  from 
that  uppermost  epiblastic  layer  of  cells,  so  early  marked  off  in 


Fig.  366. 

PlG.  365.— Outer  surface  of  human  frctal  brain  at  six  months,  showing  origin  of  prin- 
cipal fissures  (after  Sharpey  and  R.  Wagner).  F,  frontal  lobe;  P,  parietal;  0, 
occipital;  T,  temporal;  a,  a,  a,  faint  appearance  of  several  frontal  convolutions; 
g,  8,  Sylvan  fissure;  *',  anterior  division  of  same;  C,  central  lobe  of  island  of  Reil; 
r,  fissure  of  Rolando;  p,  external  perpendicular  fissure. 

Pig.  366.  Upper  surface  of  bruin  represented  in  Fig.  304  (after  Sharpey  and  R.  Wag- 
ner). 


the  blastoderm,  is  formed  *the  entire  nervous  system,  including 
centers,  nerves,  and  end  organs.  The  brain  may  be  regarded 
as  a  specially  differentiated  part  of  the  anterior  region  of  the 


THE  BRAIN. 


511 


medullary  groove  and  its  subdivisions  ;  and  the  close  relation 
of  the  eye,  ear,  etc.,  to  the  brain  in  their  early  origin,  is  not 
without  special  meaning,  while  the  more  diffused  sensory  de- 
velopments in  the  skin  connect  the  higher  animals  closely  with 
the  lower — even  the  lowest,  in  which  sensation  is  almost  wholly 
referable  to  the  surface  of  the  body. 

Without  some  knowledge  of  the  mode  of  development  of 
tbe  encephalon,  it  is  scarcely  possible  to  appreciate  that  rising 
grade  of  complexity  met  with  as  we  pass  from  lower  to  higher 
groups  of  animals,  especially  noticeable  in  vertebrates  ;  nor  is 
it  possible  to  recognize  fully  the  evidence  found  in  the  nervous 
system  for  the  doctrine  that  higher  are  derived  from  lower 
forms  by  a  process  of  evolution. 

Evolution. — The  same  law  applies  to  the  nervous  system  as 
to  other  parts  of  the  organism,  viz.,  that  tbe  individual  devel- 
opment (ontogeny)  is  a  synoptical  representation,  in  a  general 
way.  of  the  development  of  the  group  (phylogeny).  A  com- 
parison of  the  development  of  even  man's  brain  reveals  the  fact 
that,  in  its  earliest  stage,  it  is  scarcely,  if  at  all,  distinguishable 
from  that  of  any  of  the  lower  vertebrates.  There  is  a  period 
when  even  this,  the  most  convoluted  of  all  brains,  is  as  smooth 
and  devoid  of  gyri  as  the  brain  of  a  frog.     The  exti'erne  com- 


PlG.  367.— A.  brain  of  aye-aye  {Lemur);  B.  of  marmoset;  C.  of  squirrel  monkey  (Cal- 
lithri.c);  D,  of  macaque  monkey;  E,  of  gibbon;  F,  of  a  fifth-month  human  foetus 
(after  ( >wen  i.  Although  naturalists  are  agreed  that  tbe  monkey.-,  apes,  and  lemurs 
are  related,  considerable  differences  are  to  be  observed  in  their  brains.  These  fig- 
ures also  illustrate  the  remark  made  after  those  following. 


plexity  of  the  human  brain  is  referable  to  excessive  growth  of 
certain  parts,  crowding  and  alteration  of  shape,  owing  to  the 
influence  of  its  bony  case,  its  membranes,  etc. 


512 


COMPARATIVE   PHYSIOLOGY. 


It  is  evident,  from  an  inspection  of  the  cranial  cavities  of 
those  enormous  fossil  forms  that  preceded  the  higher  verte- 
brates, that  their  brains,  in  proportion  to  their  bodies,  were 
very  small,  so  that  any  variation  in  the  direction  of  increase 
in  the  encephalon — especially  the  cerebrum — must  have  given 
the  creatures,  the  subject  of  such  variation,  a  decided  advan- 
tage in  the  struggle  for  existence,  and  one  which  may  partly 
account,  perhaps,  for  the  extinction  of  those  animals  of  vast 
proportions  but  limited  intelligence.     That  the  size  of  the  brain 


Fig.  368. — A,  brain  of  a  chelonian;  B,  of  a  foetal  calf;  C,  of  a  cat.  (All  after  Gegen- 
baur.)  /,  indicates  cerebral  hemispheres  ;  //,  thalamus  ;  III,  corpora  quadri- 
gemina;  IV,  cerebellum;  V,  medulla;  st,  corpus  striatum;  /,  fornix;  h,  hippocam- 
pus; sr,  fourth  ventricle;  g,  geniculate  body;  ol,  olfactory  lobe.  It  will  be  observed 
(1)  how  the  foetal  brain  in  a  higher  animal  form  resembles  the  developed  brain  in 
a  lower  form,  and  (2)  how  certain  parts  become  crowded  together  and  covered 
over  by  more  prominent  regions,  e.  g.,  the  cerebrum,  as  we  ascend  the  animal  scale. 

as  well  as  its  quality  can  be  increased  by  use,  seems  to  have 
been  established  by  the  measurements,  at  different  periods  of 
development,  of  the  heads  of  those  engaged  in  intellectual  pur- 
suits, and  comparing  the  results  with  those  obtained  by  similar 
measurement  of  the  heads  of  those  not  thus  specially  employed. 
Of  course,  it  must  be  assumed  that  the  head  measurement  is  a 
gauge  of  the  size  of  the  brain,  which  is  approximately  true,  if 
not  entirely  so. 

Recent  investigations  seem  to  show  that  the  development 
of  the  ganglion  cells  of  the  brain  takes  place  first  in  the  me- 
dulla, next  in  the  cerebellum,  after  that  in  the  mid-brain,  and 
finally  in  the  cerebral  cortex.  Animals  most  helpless  at  birth 
are  those  with  the  least  development  of  such  cells.     The  me- 


THE  BRAIN. 


513 


dulla  may  be  regarded  in  some  sense  as  the  oldest  (phylogeneti- 
cally)  part  of  the  brain.  In  it  are  lodged  those  cells  (centers) 
which  are  required  for  the  maintenance  of  the  functions  essen- 
tial to  somatic  life.  This  may  serve  to  explain  how  it  is  that 
so  many  centers  are  there  crowded  together.  It  is  remarkable 
that  so  small  a  part  of  the  brain  should  preside  over  many  im- 
portant functions  ;  but  the  principle  of  concentration  with  pro- 
gressive development,  and  the  law  of  habit  making  automatism 
prominent,  throw  some  light  upon  these  facts,  and  especially 
the  one  otherwise  not  easy  to  understand,  that  so  much  impor- 
tant work  should  be  done  by  relatively  so  few  cells.  Possibly, 
however,  if  localization  is  established  as  fully  as  it  may  eventu- 
ally be,  this  also  will  not  be  so  astonishing. 

The  law  of  habit  has,  in  connection  with  our  psychic  life 
and  that  of  other  mammals,  some  of  its  most  striking  develop- 
ments. This  has  long  been  recognized,  though  that  the  same 
law  is  of  universal  application  to  the  functions  of  the  body  has 
as  yet  received  but  the  scantiest  acknowledgment. 

We  shall  not  dwell  upon  the  subject  beyond  stating  that  in 
our  opinion  the  psychic  life  of  animals  can  be  but  indifferently 
understood  unless  this  great  factor  is  taken  into  the  account  ; 
and  when  it  is,  much  that  is  apparently  quite  inexplicable  be- 
comes plain.  That  anything  that  has  happened  once  any- 
where in   the  vital  economy  is   liable  to   repetition  under  a 


Fig.  369. 


Fig.  370. 


Fig.  369.- 
Fig.  370.- 


-Brnin  of  cat,  seen  from  above  (after  Tiedemann). 
-Brain  of  dog,  seen  from  above  (after  Tiedemann). 


slighter  stimulus,  is  a  law  of  the  utmost  importance  in  physiol- 
ogy, psychology,  and  pathology.     The  practical  importance  of 
this,  especially  to  the  young  animal,  is  of  the  highest  kind. 
Synoptical.— There  is  as  yet  no  systematized  clear  physiology 


514  COMPARATIVE  PHYSIOLOGY. 

of  "the  brain."  We  are  conversant  with  certain  phenomena 
referable  to  this  organ  in  a  number  of  animals,  chiefly  the 
higher  mammals  ;  but  our  knowledge  is  as  yet  insufficient  to 
generalize,  except  in  the  broadest  way,  regarding  the  functions 
of  the  brain — i.e.,  to  determine  what  is  common  to  the  brains  of 
all  vertebrates  and  what  is  peculiar  to  each  group.  Referring, 
then,  to  the  higher  mammals,  especially  to  the  dog,  the  cat,  the 
monkey,  and  man,  we  may  make  the  following  statements  : 

The  medulla  oblongata  is  functionally  the  ruler  of  vegeta- 
tive life — the  lower  functions  ;  and  so  may  be  regarded  as  the 
seat  of  a  great  number  of  "  centers,"  or  collections  of  cells  with 
functions  to  a  large  degree  distinct,  but  like  close  neighbors, 
with  a  mutual  dependence. 

Phylogenetically  (ancestrally)  the  medulla  is  a  very  ancient 
region,  hence  the  explanation  apparently  of  so  many  of  its 
functions  being  common  to  the  whole  vertebrate  group. 

Parts  of  the  mesencephalon,  the  pons  Varolii,  the  optic  lobes 
or  corpora  quadrigemina,  the  crura  cerebri,  etc.,  are  not  only 
connecting  paths  between  the  cord  and  cerebrum,  but  seem  to 
preside  over  the  co-ordination  of  muscular  movements,  and  to 
take  some  share  in  the  elaboration  of  visual  and  perhaps  other 
sensory  impulses. 

The  cerebellum  may  have  many  functions  unknown  to  us. 
Its  connections  with  other  parts  of  the  nerve-centers  are  numer- 
ous, though  their  significance  is  in  great  part  unknown.  Both 
pathological  and  physiological  investigation  point  to  its  having 
a  large  share  in  muscular  co-ordination. 

It  is  certain  that  the  cerebrum  is  the  part  of  the  brain  essen- 
tial for  all  the  higher  psychic  manifestations  in  the  most  ad- 
vanced mammals  and  in  man. 

The  preponderating  development  of  man's  cerebrum  ex- 
plains at  once  his  domination  in  the  animal  world,  his  power 
over  the  inanimate  forces  of  Nature,  and  his  peculiar  infirmities, 
tendencies  to  a  certain  class  of  diseases,  etc., — in  a  word,  man  is 
man,  largely  by  virtue  of  the  size  and  peculiarities  of  this  part 
of  his  brain. 

Modern  research  has  made  it  clear  also  that  there  is  a  "  pro- 
jection "  of  sensory  and  motor  phenomena  in  the  cerebral  cor- 
tex ;  in  other  words,  that  there  are  sensory  and  motor  centers 
in  the  sense  that  in  the  cortex  there  are  certain  cells  which  have 
an  important  share  in  the  initiation  of  motor  impulses,  and 
others  employed  in  the  final  elaboration  of  sensory  ones. 


THE  BRAIN.  515 

It  is  even  yet  premature  to  dogmatize  in  regard  to  the  site 
of  these  centers;  especially  are  we  not  ready  for  large  generali- 
zations. In  man  the  convolutions  around  the  fissure  of  Rolando 
constitute  the  motor  area  hest  determined. 

The  whole  subject  of  cortical  localization  requires  much  ad- 
ditional study,  especially  by  the  comparative  method  in  the 
widest  sense — i.  e.,  by  a  comparison  of  the  results  of  operative 
procedure  in  a  variety  of  groups  of  animals,  and  the  results  of 
clinical,  pathological,  physiological,  and  psychological  investi- 
gation. Especially  must  allowance  be  made  for  differences  to 
be  observed,  both  for  the  group  and  the  individual  ;  and  also 
for  the  influence  which  one  region  exerts  over  another.  Be- 
tween the  weight  of  the  cerebrum,  the  extent  of  its  cortical 
surface,  and  psychic  power,  there  is  a  general  relationship. 


GENERAL   REMARKS   ON   THE   SENSES. 


Oue  studies  in  embryology  have  taught  us  that  all  the  vari- 
ous forms  of  end-organs  are  developed  from  the  epiblast,  and 
so  may  be  regarded  as  modified  epithelial  cells,  with  which  are 
associated  a  vascular  and  nervous  supply.  These  end-organs 
are  at  once  protective  to  the  delicate  nerves  which  terminate  in 


Fig.  371.— Papillae  of  skin  of  palm  of  hand  (after  Sappey).    A  vascular  network  in  all 
cases,  and  in  some  nerves  and  tactile  corpuscles,  enter  the  papillae. 

them,  and  serve  to  convey  to  the  latter  peculiar  impressions 
which  are  widely  different  in  most  instances  from  those  result- 
ing from  the  direct  contact  of  the  nerve  with  the  foreign  body, 
All  are  acquainted  with  the  fact  that,  when  the  epithelium  is 
removed,  as  by  a  blister,  we  no  longer  possess  tactile  sensibility 
of  the  usual  kind,  and  experience  pain  on  contact  with  objects ; 
in  a  word,  the  series  of  connections  necessary  to  a  sense-percep- 
tion is  broken  at  the  commencement. 

Seeing  that  all  the  end-organs  on  the  surface  of  the  body 
have  a  common  origin  morphologically,  it  would  be  reasonable 
to  expect  that  the  senses  would  have  much  in  common,  espe- 
cially when  these  organs  are  all  alike  connected  with  central 
nervous  cells  by  nerves.     As  a  matter  of  fact,  such  is  the  case, 


GENERAL   REMARKS   ON  THE  SENSES. 


517 


and  in  every  instance  we  can  distinguish  between  sensory  im- 
pulses generated  in  the  end-organ,  conveyed  by  a  nerve  inward, 
and  those  in  the  cells  of  these  central  nervous  systems,  giving 
rise  to  certain  molecular  changes 
which  enable  the  mind  or  the 
ego  to  have  a  perception  proper ; 
which,  when  taken  in  connec- 
tion with  numerous  past  experi- 
ences of  this  and  other  senses, 
furnishes  the  material  for  a  sen- 
sory judgment. 

The  chief  events  are,  after 
all,   internal,   and   hence    it    is 


Fig.  372. 


Fig  373. 


Fig.  372.— Corpuscle  of  Vater  (after  Sappey). 

Fig.  373. — End-bulbs  (corpuscles)  of  Krause  (after  Luddon).  A.  from  conjunctiva  ot 
man;  B,  from  conjunctiva  of  calf.  It  may  be  noticed  that  in  all  these  cases  the 
nerve  loses  its  non-essential  parts  before  entering  the  corpuscle. 


found  that  the  higher  in  the  scale  the  animal  ranks,  the  more  de- 
veloped its  nervous  centers,  especially  its  brain,  and  the  more  it 
is  able  to  capitalize  its  sensory  impulses ;  also  the  greater  the  de- 
gree of  possible  improvement  by  experience,  a  difference  well 
seen  in  blind  men  whose  ability  to  succeed  in  life  without  vision 
is  largely  in  proportion  to  their  innate  and  acquired  mental 
powers.     Inasmuch  as  all  cells  require  rest,  one  would  expect 


518 


COMPARATIVE   PHYSIOLO  -Y. 


that  under  eonstaut  stimulation  fatigue  w.  raid  soon  result  ana 
perceptions  be  imperfect     Hence  it  happens  that  all  the  senses 

fail    when    exercised. 
G 
\ 


even  for  but  a  short 
period,  without  change 

of  stimulus  lead::  _ 
alteration  of  condition 
in  the  central  cells, 
The  change  need  not 
be  one  of  entire  rest. 
but  merely  a  new  form 
of  exercise.  Hence  the 
freshness  experienced 
by  a  change  of  view  on 
passing  through  beau- 
tiful scenery. 

Exhaustion  may 
not  he  confined  wholly 
to   the   central  nerve- 

Fig.  374.— Nerves  with  sranglion  cells  {G)  beneath  a    cells,  but  there  Can  be 

$&?Jw*'-  fr°m  Skin  °f  "m  arthropod  little  doubt  that  they 

are  the  most  affected. 
Since  also  there  must  be  a  certain  momentum,  so  to  speak,  to 

molecular  activity,  it  is  not  surprising  that  we  find  that  the 
sensation  outlasts  the  stimulus  for  a  brief  period:  and  this  ap- 
plies to  all  the  senses,  and  necessarily  determines  the  rapidity 
with  which  the  successive  stimuli  may  follow  each  other  with- 
out causing  a  blending  of  tne  sensations. 


Thus,  then,  in  every  sense 
organ  in  which  the  chain  of  pi 
nerve  through  which  (3    the  central  near 
and  we  may  speak,  therefore,  of  (1)  senso 
sensations,  when  these  give  rise   t<->  atf'ect 
nervous  cells  resulting  in    1     perceptions 
when  we  take  into  account  the  psychic  i  r 
nature  of  cell-life  generally,  we  must  reco£ 
sity  of  the  stimulus  ry  to  arouse  a  sensation  and  a  limit 

within  which  alone  we  have  power  to  discriminate  (range  of 
stimulation  and  perception):  and  also  a  limit  to  the  ra 
with  which  stimuli  may  succeed  each  other  to  any  advantage, 
to  give  ris  rsens         is;  and  a  limit  to  the  endur- 

ance of  the  apparatus  in  good  working  condition  corresponding 


must  recognize  1<  an  end- 
ses  begins;  (2)  a  conducting 
e-cells  are  affected ; 

y  impulses  and     2 
ons  of  the  central 

and     •„'     jui  a. 
e-^e> :  and.  from  the 
a  certain  inten- 


GENERAL   REMARKS  ON  THE  SENSES.  519 

to  clear  mental  perceptions,  together  with  the  value  of  past  ex- 
perience in  the  interpretation  of  our  sensations.  A  man  can 
necessarily  have  positive  knowledge  only  of  his  own  conscious- 
ness ;  but  he  infers  similarity  of  conscious  states  by  likeness  in 
action  and  expression  in  his  fellows.  It  is  by  an  analogous 
process  and  by  such  alone  that  we  can  draw  any  conclusions  in 
regard  to  the  sensations  of  the  lower  animals.  The  presence  of 
structures,  undoubtedly  sensory,  in  them  is  fairly  good  evidence 
that  their  sensations  resemble  ours  when  similar  organs  are  em- 
ployed. However,  this  does  not  absolutely  follow;  and  the 
whole  subject  of  the  senses  of  animals  incapable  of  articulate 
speech  is  beset  witb  great  difficulties.  It  only  remains  for  us  to 
set  forth  what  is  known  regarding  man,  assuming  that  at 
least  much  of  it  applies  to  our  domestic  animals.  Patient 
thoughtful  observation  will  in  time  place  the  subject  in  a  better 
position. 


THE   SKIN  AS   AN   ORGAN   OF   SENSE. 


Bearing  in  mind  that  all  the  sensory  organs  originate  in 
the  ectoderm,  we  find  in  the  skin  even  of  the  highest  animals 
the  power  to  give  the  central  nervous  system  such  sense-im- 
pressions as  bear  a  relation  to  the  original  undifferentiated 
sensations  of  lower  forms  as  derived  from  the  general  surface 
of  the  body,  but  with  less  of  specialization  than  is  met  with  in 
the  sense  of  hearing  and  vision,  so  that  it  is  possible  to  under- 
stand how  it  is  that  the  skin  must  be  regarded  not  only  as  the 
original  source  of  sensory  impulses  for  the  animal  kingdom, 
but  why  it  still  remains  perhaps  the  most  important  source  of 
information  in  regard  to  the  external  world,  and  the  condition 
of  our  own  bodies;  for  it  must  be  remembered  that  the  data 
afforded  for  sensory  judgments  by  all  the  other  senses  must 
be  interpreted  in  the  light  of  information  supplied  by  the  skin. 
We  really  perceive  by  the  eye  only  retinal  images.  The  dis- 
tance, position,  shape,  etc.,  of  objects  are  largely  determined  by 
feeling  them,  and  thus  associating  with  a  certain  visual  sensa- 
tion others  derived  from  the  skin  and  the  muscles,  which  latter 
are,  however,  generally  also  associated  with  tactile  sensations. 

It  is  recorded  of  those  blind  from  birth  that,  when  restored 
to  sight  by  surgical  operations,  they  find  themselves  quite  un- 
able to  interpret  their  visual  sensations;  or,  in  other  words, 
seeing  they  do  not  understand,  but  must  leaim  by  the  other 
senses,  especially  tactile  sensibility,  what  is  the  real  nature  of 
the  objects  that  form  images  on  their  retinae.  All  objects  seen 
appear  to  be  against  the  eyes,  and  any  idea  of  distance  is  out  of 
the  question. 

Special  forms  of  end-organs  are  found  scattered  over  the 
skin,  mucous  and  serous  surfaces  of  the  body,  such  as  Pacinian 
corpuscles,  touch-corpuscles,  end-bulbs,  etc. ;  while  in  lower 
forms  of  vertebrates  many  others  are  found  in  parts  where  sen- 
sibility is  acute.     There  seems  to  be  little  doubt  that  these  are 


THE  SKIN  AS  AN   ORGAN  OF   SENSE.  521 

all  concerned  with  the  various  sensory  impulses  that  originate 
in  the  parts  where  they  are  found,  but  it  is  not  possible  at  pres- 
ent to  assign  definitely  to  each  form  its  specific  function. 

It  has  been  contended  that  the  various  specific  sensations 
of  taste,  as  bitter,  sweet,  etc.,  are  the  result  of  impulses  con- 
veyed to  the  central  nervous  system  by  fibers  that  have  this 
function,  and  no  other;  and  a  like  view  has  been  maintained 
for  those  different  sensations  that  originate  from  the  skin. 
For  such  a  doctrine  there  is  a  certain  amount  of  support  from 
experiment  as  well  as  analogy ;  but  the  more  closely  the  subject 
is  investigated  the  more  it  appears  that  the  complexity  of  our 
sensations  is  scarcely  to  be  explained  in  so  simple  a  way  as 
many  of  these  theories  would  lead  us  to  believe.  Whether 
there  are  nerve-fibers  with  functions  so  specific,  must  be  re- 
garded as  at  least  not  yet  demonstrated. 

Let  us  now  examine  into  the  facts.  What  are  the  different 
sensations,  the  origin  of  which  must  be  in  the  first  instance, 
sought  in  the  skin,  as  the  impulses  aroused  in  some  form  of 
end-organ  or  nerve-termination  ? 

Suppose  that  one  blindfolded  lays  his  left  hand  and  arm 
on  a  table,  and  a  piece  of  iron  be  placed  on  the  palm  of  his 
hand,  he  may  be  said  to  be  conscious  of  the  nature  of  the  sur- 
face, whether  rough  or  smooth,  of  the  form,  of  the  size,  of  the 
weight,  and  of  the  temperature  of  the  body:  in  other  words, 
the  subject  of  the  experiment  has  sensations  of  pressure,  of 
tactile  sensibility,  and  of  temperature  at  least,  if  not  also  to 
some  extent  of  muscular  sensibility.  But  if  the  right  hand  be 
used  to  feel  the  object  its  form  and  surface  characters  can  be 
much  better  appreciated ;  while,  if  the  body  be  poised  in  the 
hand,  a  judgment  as  to  its  weight  can  be  formed  with  much 
greater  accuracy.  The  reason  of  the  former  is  to  be  sought  in 
the  fact  that  the  finger-tips  are  relatively  very  sensitive  in 
man,  and  that  from  experience  the  mind  has  the  better  learned 
to  interpret  the  sensory  impulses  originating  in  this  quarter; 
which  again  resolves  itself  into  the  particular  condition  of  the 
central  nerve-cells  associated  with  the  nerve-fibers  that  convey 
inward  the  impulses  from  those  regions  of  the  skin.  Mani- 
festly if  there  be  a  sense  referable  to  the  muscles  (muscular 
sense)  at  all,  when  they  are  contracted  at  will  the  impression 
must  be  clearer  than  when  they  but  feebly  respond  to  the  mere 
pressure  of  some  body. 


522  COMPARATIVE  PHYSIOLOGY. 

PRESSURE    SENSATIONS. 

1.  There  is  a  relation  between  the  intensity  of  the  stimulus 
and  the  sensation  resulting,  and  this  limit  is  narrow.  The 
greater  the  stimulus  the  more  pronounced  the  sensation,  though 
ordinary  sensibility  soon  passes  into  pain.  2.  The  law  of  con- 
trast may  be  illustrated  by  passing  the  finger  up  and  down  in  a 
vessel  containing  mercury,  when  the  pressure  will  be  felt  most 
distinctly  at  the  point  of  contact  of  the  fluid.  3.  Pressure  is 
much  better  estimated  by  some  parts  than  others ;  bence  the  use 
of  the  employment  of  those  to  so  large  an  extent. 

THERMAL   SENSATIONS. 

1.  The  law  of  contrast  is  well  illustrated  by  this  sense ;  in 
fact,  the  temperature  of  a  body  exactly  the  same  as  that  of  the 
part  of  the  skin  applied  to  it  can  scarcely  be  estimated  at  all. 
The  first  plunge  into  a  cold  bath  gives  the  impression  that  the 
water  is  much  colder  than  it  seems  in  a  few  seconds  after,  when 
the  temperature  has  in  reality  changed  but  little ;  or,  perhaps, 
the  subject  may  be  better  illustrated  by  dipping  one  hand  into 
wanner  and  the  other  into  colder  water  than  that  to  be  ad- 
judged. The  sample  feels  colder  than  it  really  is  to  the  hand 
that  has  been  in  the  warm  water,  and  warmer  than  it  is  to  the 
other.  2.  The  limit  within  which  we  can  discriminate  is  at  most 
small,  and  the  nicest  determinations  are  made  within  about  27° 
and  33°  C. — i.  e.,  not  far  from  the  normal  temperature  of  the 
body.  3.  Variations  for  the  different  parts  of  the  skin  are 
easily  ascertained,  though  they  do  not  always  correspond  to 
those  most  sensitive  to  changes  in  pressure.  The  cheeks,  lips, 
and  eyelids  are  very  sensitive  to  pressure. 

Recent  investigations  have  revealed  the  fact  that  there  are  in 
the  human  skin  "  pressure-spots,''  and  "  cold-spots,"  and  "  heat- 
spots"— i.  e.,  the  skin  may  be  mapped  out  into  very  minute 
areas  which  give  when  touched  a  sensation  of  pressure  different 
from  that  produced  by  the  same  stimulus  in  the  intermediate 
regions ;  and  in  like  manner  are  there  areas  which  are  sensitive 
to  warm  and  to  cold  bodies  respectively,  but  not  to  both ;  and 
these  do  not  correspond  with  the  pressure-spots,  nor  to  those 
that  give  rise  when  touched  to  the  sensation  of  pain. 

It  has  been  shown,  also,  that  the  extent  of  the  area  of  skin 
stimulated  detennines  to  a  large  degree  the  quality  of  the  re- 


THE  SKIN  AS  AN  ORGAN  OF  SENSE.  523 

suiting  sensation.  Thus,  the  temperature  of  a  fluid  does  not 
seem  the  same  to  a  finger  and  the  entire  hand.  This  fact  is  not 
irreconcilable  with  the  existence  of  the  various  kinds  of  ther- 
mal spots,  referred  to  above,  but  it  does  re-enforce  the  view  we 
are  urging  of  the  complexity  of  those  sensations  which  seem  to 
us  to  form  simple  wholes — as,  indeed,  they  do — just  as  a  piece 
of  cloth  may  be  made  up  of  an  unlimited  number  of  different 
kinds  of  threads. 

TACTILE   SENSIBILITY. 

As  a  matter  of  fact,  one  may  learn,  by  using  a  pair  of  com- 
passes, that  the  different  parts  of  the  surface  of  our  bodies  are 
not  equally  sensitive  in  the  discrimination  between  the  contact 
of  objects — i.  e.,  the  judgment  formed  as  to  whether  at  a  given 
instant  the  skin  is  being  touched  by  one  or  two  points  is  de- 
pendent on  the  part  of  the  body  with  which  the  points  are 
brought  into  contact. 

Certain  it  is  that  exercise  of  these  and  all  the  senses  greatly 
improves  them,  though  it  is  likely  that  such  advance  must  be 
referred  rather  to  the  central  nerve-cells  than  to  the  peripheral 
mechanism. 

We  practically  distinguish  between  a  great  many  sensations 
that  we  can  neither  analyze  nor  describe,  though  the  very 
variety  of 'names  suffices  to  show  how  much  our  interpretation 
of  sense  depends  on  past  experience. 

Mammals  are  always  able  to  define  the  part  of  their  bodies 
touched,  and  with  great  accuracy,  no  doubt,  owing  to  the  simul- 
taneous use  in  the  early  months  and  years  of  life  of  vision  and 
the  senses  resident  in  the  skin. 

An  impression  made  on  the  trunk  of  a  nerve  is  referred  to 
the  peripheral  distribution  of  that  nerve  in  the  skin ;  thus,  if 
the  elbow  be  dipped  in  a  freezing  mixture,  the  skin  around  the 
joint  will  experience  the  sensation  of  cold,  but  a  feeling  of  pain 
will  be  referred  to  the  distribution  of  the  ulnar  nerve  in  the 
hand  and  arm.  The  same  principle  is  illustrated  by  the  com- 
mon experience  of  the  effects  of  a  blow  over  the  ulnar  nerve, 
the  pain  being  referred  to  the  peripheral  distribution  ;  also  by 
the  fact  that  pain  in  the  stump  of  an  amputated  limb  is  thought 
to  arise  in  the  missing  toes,  etc. 


524  COMPARATIVE  PHYSIOLOGY. 

THE    MUSCULAR    SENSE. 

Every  one  must  be  aware  how  difficult  it  is  to  regulate  his 
movements  when  the  limbs  are  cold  or  otherwise  deadened  in 
sensibility.  We  know  too  that,  in  judging  of  the  muscular 
effort  necessary  to  be  put  forth  to  accomplish  a  feat,  as  throw- 
ing a  ball  or  lifting  a  weight,  we  judge  by  our  past  experience. 
It  is  ludicrous  to  witness  the  failure  of  an  individual  to  pick  up 
a  mass  of  metal  which  was  mistaken  for  wood.  In  these  facts 
we  recognize  that  in  the  successful  use  of  the  muscles  we  are 
dependent,  not  alone  on  the  sensations  derived  from  the  skin, 
but  also  from  the  muscles  themselves.  True,  the  muscles  are 
not  very  sensitive  to  pain  when  cut;  it  does  riot,  however,  fol- 
low that  they  may  not  be  sensitive  to  that  different  effect,  their 
own  contraction.  Whether  the  numerous  Pacinian  bodies 
around  joints,  or  the  end-organs  of  the  nerves  of  muscles  are 
directly  concerned,  is  not  determined. 

Pathological. — The  teaching  of  disease  is  plainly  indicative 
of  the  importance  of  sensations  derived  both  from  the  skin  and 
the  muscles  for  co-ordination  of  muscular  movements. 

In  locomotor  ataxy,  in  which  the  power  of  muscular  co- 
ordination is  lost  to  a  large  extent,  the  lesions  are  in  the  pos- 
terior columns  of  the  spinal  cord,  or  the  posterior  roots  of  the 
nerves,  or  both,  and  these  are  the  parts  involved  in  the  trans- 
mission of  afferent  impulses. 

Comparative. — The  more  closely  the  higher  vertebrates  are 
observed,  the  more  convinced  does  one  become  that  those  sen- 
sory judgments,  based  upon  the  information  derived  from  the 
skin  and  muscles,  which  they  are  constantly  called  upon  to  form 
are  in  extent,  variety,  and  perfection  scarcely  if  at  all  surpassed 
by  those  of  man.  Of  course,  sensory  data  in  man,  with  his  ex- 
cessive cerebral  development,  may  by  associations  in  his  expe- 
rience be  worked  up  into  elaborate  judgments  impossible  to  the 
brutes,  but  we  now  refer  to  the  judgments  of  sense  in  them- 
selves. 

The  lips,  the  ears,  the  vibrissa?  or  stiff  hairs,  especially  about 
the  lips,  the  nose,  in  some  cases  the  paws,  all  afford  delicate  and 
extensive  sensory  data. 

It  is  a  remarkable  fact  that  the  most  intelligent  of  the 
groups  of  animals  have  these  sensory  surfaces  well  developed, 
as  witness  the  elephant  with  his  wonderful  trunk,  the  band  of 
the  monkey,  and  the  paws  and  vibrissa?  of  the  cat  and  dog  tribe. 


THE  SKIN  AS  AN  ORGAN   OF  SENSE.  525 

On  the  other  hand,  the  groups  with  hoofs  are  notably  inferior 
in  the  mental  scale.  When  we  pass  to  the  lower  forms  of  in- 
vertebrates the  appreciation  of  vibrations  of  the  air  or  water 
in  which  they  live,  of  its  temperature,  of  its  pressure,  etc.,  must 
be  considerable  to  enable  them  to  adapt  themselves  to  a  suitable 
environment. 

We  have  not  spoken  of  sensations  derived  from  the  internal 
organs  and  surfaces.  These  are  ill-defined,  and  we  know  them 
mostly  either  as  a  vague  sense  of  comfort  or  discomfort,  or  as 
actual  pain.  We  are  quite  unable  to  refer  them  at  present  to 
special  forms  of  end-organs.  They  are  valuable  as  reports  and 
warnings  of  the  animal's  own  conditions. 

After-impressions  ("  after-images")  of  all  the  senses  referred 
to  exist,  mostly  positive  in  nature— i.  e.,  the  sensation  remains 
when  the  stimulus  is  withdrawn. 

Synoptical. — The  information  derived  from  the  skin  in  man 
and  the  other  higher  vertebrates  relates  to  sensations  of  press- 
ure, temperature,  touch,  and  pain.  The  muscles  also  supply 
information  of  their  condition.  In  how  far  these  are  referable 
to  certain  end-organs  in  the  skin  is  uncertain.  There  are  der- 
mal areas  that  give  rise  to  the  sensations  of  heat,  cold,  pressure, 
and  pain.  Whether  these  are  connected  with  nerve-fibei's  that 
convey  no  other  forms  of  impulses  than  those  thus  arising  is 
undetermined . 

In  all  these  senses  the  laws  of  contrast,  duration  of  the  im- 
pression, limit  of  discrimination,  etc.,  hold. 

The  judgments  based  on  sensations  derived  from  the  skin 
are  syntheses  or  the  result  of  the  blending  of  many  component 
sensations  simultaneous  in  origin.  All  our  sensory  judgments 
are  very  largely  dependent  on  our  past  experience. 


VISION. 


Light  and  vision  are  to  some  degree  correlatives  of  each 
other.  Light  is  supposed  to  have  as  its  physical  basis  the  vibra- 
tions of  an  imponderable  ether.  Such  is,  however,  to  a  non- 
seeing  animal  as  good  as  non-existent,  so  that  we  may  look  at 


15  13  16 


Fig.  37.").— Eye  partially  dissected  (after  Sappeyl.  1.  optic  nerve;  2,  3,  4,  sclerotic  dis- 
sected back  so  as  to  uncover  the  choroid  coat;  5,  cornea,  divided  and  folded  back 
with  sclerotic  coat;  6,  canal  of  Schlemm;  7,  external  surface  of  choroid,  traversed 
by  one  of  the  long  ciliary  arteries  and  by  ciliary  nerves;  8.  central  vessel,  into 
which  the  vam  vorticoea  empty;  9,  10.  choroid  zone;  11,  ciliary  nerves;  12,  long 
ciliary  artery;  13,  anterior  ciliary  arteries;  14,  iris;  15,  vascular  circle  of  iris;  16, 
pupil. 

this  subject  either  with  the  eyes  of  the  physiologist  or  the  phys- 
icist, according  as  we  regard  the  cause  of  the  effects  or  the 
latter  and  their  relations  to  one  another.  It  is,  however,  im- 
possible to  understand  the  physiology  of  vision  without  a 
sound  knowledge  of  the  anatomy  of  the  eye,  and  an  apprehen- 


VISION. 


527 


sion  of  at  least  some  of  the  laws  of  the  science  of  optics.     The 
student  is,   therefore,  recommended   to   learn  practically  the 


SUPERIOR  RECTUS 


CHOROID 


OPTIC  NERVE 


-INFERIOR  RECTUS 
Fig.  376.— Section  of  human  eye,  somewhat  diagrammatic  (after  Flint) 

coarse  and  microscopic  structure  of  the  eye  in  detail.  The  eyes 
of  mammals  are  sufficiently  alike  to  make  the  dissection  of  any 
of  them  profitable.  Bullocks'  eyes  are  readily  obtainable,  and 
from  their  large  size  may  be  used  to  advantage.  We  recom- 
mend one  to  be  boiled  hard,  another  to  be  frozen,  and  sections 
in  different  meridians  to  be  made,  especially  one  axial  vertical 
longitudinal  section.  Other  specimens  may  be  dissected  with 
and  without  the  use  of  water. 

Assuming  that  some  such  work  has  been  done,  and  that  the 
student  has  become  quite  familiar  with  the  general  structure 
of  the  eye,  we  call  attention  specially  to  the  strength  of  the 
sclerotic  coat ;  the  great  vascularity  of  the  choroid  coat  and  its 
terminal  ciliary  processes,  its  pigmented  character  adapting  it 
for  the  absorption  of  light;  the  complicated  structure  and  pro- 
tected position  of  the  retinal  expansion.     It  may  be  said  that 


528 


COMPARATIVE   PHYSIOLOGY. 


the  whole  eye  exists  for  the  retina,  and  that  the  entire  mechan- 
ism besides  is  subordinated  to  the  formation  of  images  on  this 


Fig.  377. — Certain  parts  of  eye.  1  x  10.  (After  Sappey.)  1,  1,  crystalline  lens;  2, 
hyaloid  membrane;  3,  zonule  of  Zinn;  4,  iris;  5,  a  ciliary  process;  6,  radiating 
fibers  of  ciliary  muscle;  7,  section  of  circular  portion  of  ciliary  muscle;  8,  venous 
plexus  of  ciliary  muscle;  9, 10,  sclerotic  coat;  11, 12,  cornea;  13,  epithelial  layer  of 
cornea;  14.  Descemet's  membrane;  15,  pectinate  ligament  of  iris;  16,  epithelium 
of  membrane  of  Descemet;  17,  union  of  sclerotic  coat  with  cornea;  18,  section  of 
canal  of  Schlemm. 

nervous  expansion.  The  eye  of  the  mammal  may  be  regarded 
as  an  arrangement  of  refracting  media,  protected  by  coverings, 
with  a  window  for  the  admission  of  light,  a  curtain  regulating 
the  quantity  admitted;  a  sensitive  screen  on  which  the  images 
are  thrown ;  surfaces  for  the  absorption  of  superfluous  light  ; 
apparatus  for  the  protection  of  the  eye  as  a  whole,  and  for  pre- 
serving exposed  parts  moist  and  clean. 

Embryological. — We  have  already  learned  that  the  first  indi- 
cation of  the  eye  is  the  formation  of  the  optic  vesicle,  an  out- 
growth from  the  first  cerebral  vesicle.     This  optic  vesicle  be- 


VISION. 


329 


comes  more  contracted  at  the  base,  and  the  optic  stalk  remains 
as  the  optic  nerve. 

At  an  early  stage  of  development  (second  or  third  day  in  the 
chick)  the  outer  portion  of  the  optic  vesicle  is  pushed  inward, 


Fig.  379. 


Fig.  378. 


Fig.  378. — Section  through  head  of  chick  on  third  day,  showing  origin  of  eye  (after 
Yeo).  a,  epiblast  undergoing  thickening  to  form  lens;  o,  optic  vesicle;  Vx,  first 
cerebral  vesicle;  V2,  posterior  cerebral  vesicle.  It  will  be  observed  that  the  retina 
is  already  distinctly  indicated. 

Fig.  379. — Later  stages  in  development  of  eye  (after  Cardiat).  a,  epiblast;  c,  develop- 
ing lens;  o,  optic  vesicle. 

so  that  the  cavity  is  almost  obliterated;  the  anterior  portion, 
becoming  thickened,  ultimately  forms  the  retina  proper,  while 
the  posterior  is  represented  by  the  tesselated  pigment  layer  of 
the  choroid. 

As  this  retinal  portion  breaks  away  from  the  superficial 
epithelium,  the  latter  foi'ms  an  elliptical  mass  of  cells,  the  future 
lens,  the  changes  of  which  in  the  formation  of  the  cells  peculiar 
to  the  lens  illustrate  to  how  great  lengths  differentiation  in 
structue  is  carried  in  the  development  of  a  single  organ.  It 
will  thus  be  seen  that  the  most  essential  parts  of  the  eye,  the 
optic  nerve,  the  retina,  and  the  crystalline  lens,  are,  according 
to  a  general  law,  the  earliest  marked  out.  The  cornea,  the  iris, 
the  choroid,  the  vascular  supply,  the  sclerotic,  etc.,  are  all  sec- 
ondary in  importance  and  in  formation  to  these,  and  are  derived 
from  the  mesoblast,  while  the  essential  structures  are  traceable, 
like  the  nervous  system  itself,  to  the  epiblastic  layer. 

Any  act  of  perfect  vision  in  a  mammal  may  be  shown  to 
consist  of  the  following:  (1)  The  focusing  of  rays  of  light  from 
34 


530 


COMPARATIVE   PHYSIOLOGY. 


an  object  on  the  retina,  so  as  to  form  a  well-defined  image;  (2) 
the  conduction  of  the  sensory  impulses  thus  generated  in  the 


Fig.  380.—  More  advanced  stage  of  development  of  eye  (after  Cardiat).  «,  epithelial 
cells  forming  lens,  now  much  altered;  b,  lens  capsule;  c,  cutaneous  tissue  about 
to  form  conjunctiva;  d,  e,  two  layers  of  optic  vesicle,  now  folded  back  and  form- 
ing retina;  /,  mucous  tissue  forming  vitreous  humors;  g,  intercellular  substance; 
h,  developing  optic  nerve;  i,  nerve-fibers  entering  retina. 

retina  by  the  optic  nerve  inward  to  certain  centers ;  and  (3)  the 
elaboration  of  these  data  in  consciousness. 

We  thus  have  the  formation  of  an  image — a  physical  pro- 
cess; sensation,  perception,  and  judgment— physiological  and 
psychical  processes. 

In  the  natural  order  of  things  we  must  discuss  first  those 
arrangements  which  are  concerned  with  the  focusing  of  light — 
i.  e.,  the  formation  of  the  image  on  the  retinal  screen. 


VISION. 


531 


DIOPTRICS   OF   VISION. 

One  of  the  most  satisfactory  methods  of  ascertaining  that 
the  eye  does  form  images  of  the  objects  in  the  field  of  vision 
is  to  remove  the  eye  of  a  recently  killed  albino  rabbit.  On 
holding  up  before  such  an  eye  any  small  object,  as  a  pair  of 
forceps,  it  may  be  readily  observed  that  an  inverted  image  of 
the  object  is  formed  on  the  back  of  the  eye  {fundus).  If, 
however,  the  lens  be  removed  from  such  an  eye,  no  image  is 
formed.  If  the  lens  be  itself  held  behind  the  object,  an  in- 
verted image  will  be  thrown  upon  a  piece  of  paper  held  at  a 
suitable  (its  focal)  distance.  By  substituting  an  ordinary  bicon- 
vex lens,  the  same  effect  follows.  It  thus  appears,  then,  that 
the  lens  is  the  essential  part  of  the  refracting  media,  though 
the  aqueous  and  vitreous  humors  and  the  cornea  are  also  focus- 
ing mechanisms. 

In  the  actual  human  eye  the  focus  must  correspond  with  the 
fovea  of  the  retina  if  a  distinct  image  is  to  be  formed. 


I  II 

Fkj.  381.— Refraction  by  convex  lenses  (after  Flint  and  Weinhokl).  The  lens  may  be 
assumed  to  consist  of  a  series  of  lenses  (II  in  figure),  for  the  sake  of  simplicity. 
though,  of  course,  this  is  not  strictly  accurate. 

It  will  appear  that  we  may  represent  the  eye  as  reduced  to 
the  lens  and  the  retina.  The  experiments  referred  to  above 
will  convince  the  student  that  such  is  the  case. 


532  COMPARATIVE   PHYSIOLOGY. 

ACCOMMODATION   OF   THE    EYE. 

Using  the  material  already  referred  to,  the  student  may 
observe  that,  with  the  natural  eye  of  the  albino  rabbit,  its 
lens  (or  better  that  of  a  bullock's  eye,  being  larger),  or  a  bi- 
convex lens  of  glass,  there  is  only  one  position  of  the  instru- 
ments and  objects  which  will  produce  a  perfectly  distinct  image. 
If  either  the  eye  (retina),  the  lens,  or  the  object  be  shifted,  in- 
stead of  a  distinct  image,  a  blurred  one,  or  simply  diffusion- 
circles,  appear. 

A  photographer  must  alter  either  the  position  of  the  object 
or  the  position  of  his  lens  when  the  focus  is  not  perfect.  The 
eye  may  be  compared  to  a  camera,  and  since  the  retina  and 
lens  can  not  change  position,  either  the  shape  of  the  lens  must 
change  or  the  object  assume  a  different  position  in  space.  As 
a  matter  of  fact,  any  one  may  observe  that  he  can  not  see 
objects  distinctly  within  a  certain  limit  of  nearness  to  the  eye, 
known  as  the  near  point  (punctum  proximum)  ■;  while  he  be- 
comes conscious  of  no  effect  referable  to  the  eye  until  objects 
approach  within  about  sixty-five  to  seventy  yards.  Beyond  the 
latter  distance  objects  are  seen  clearly  without  any  effort. 

There  are  many  ways  in  which  we  may  be  led  to  realize 
these  truths :  1.  When  one  is  reading  a  printed  page  it  is  only 
the  very  few  words  to  which  the  eyes  are  then  specially  di- 
rected that  are  seen  clearly,  the  rest  of  the  page  appearing 
blurred ;  and  the  same  holds  for  the  objects  in  any  small  room. 
We  speak  of  picking  out  an  acquaintance  in  an  audience  or 
crowd,  which  implies  that  each  of  the  individuals  composing 
the  throng  is  not  distinctly  seen  at  the  same  time.  2.  If  an  ob- 
server hold  up  a  finger  before  his  eyes,  and  direct  bis  gaze  into 
the  distance  (relax  his  accommodation),  presently  he  will  be- 
hold a  second  shadowy  finger  beside  the  real  one — i.  e.,  he  sees 
double;  his  eyes  being  accommodated  for  the  distant  objects, 
can  not  adapt  themselves  at  the  same  time  for  near  ones. 

In  what  does  accommodation  consist  ?  From  experiments 
it  has  been  concluded  that  accommodation  consists  essentially 
in  an  alteration  of  the  convexity  of  the  anterior  surface  of  the 
lens. 

This  change  in  the  shape  of  the  lens  is  accomplished  as 
follows  :  The  lens  is  naturally  very  elastic  and  is  kept  in  a  par- 
tially compressed  condition  by  its  capsule,  to  which  is  attached 
the  suspensory  ligament  which  has  a  posterior  attachment  to 


VISION. 


533 


the  choroid  and  ciliary  processes.     When  the  ciliary  muscle, 
which  operates  from  a  fixed  point  the  corneo-sclerotic  junction, 


Fig.  382. — Illustrates  mechanism  of  accommodation  (after  Fick).  The  left  side  de- 
picts the  relation  of  parts  during  the  passive  condition  of  the  eye  (negative  accom- 
modation, or  accommodation  for  long  distances);  the  right  side,  that  for  near  ob- 
jects. 

pulls  upon  the  choroid,  etc.,  it  relaxes  the  suspensory  ligament; 
hence  the  lens,  not  being  pressed  upon  in  front  as  it  is  from 
behind  by  the  vitreous  humor  (invested  by  its  hyaloid  mem- 
brane), is  free  to  bulge  and  so  increase  its  refractive  power. 
The  nearer  an  object  approaches  the  eye,  the  greater  the  diver- 
gence of  the  rays  of  light  proceeding  from  it,  and  hence  the 
necessity  for  greater  focusing  power  in  the  lens. 

If  an  animal  be  observed  closely  when  looking  from  a  remote 
to  a  near  object,  it  may  be  noticed  that  the  eyes  turn  inward — 
i.e.,  the  visual  axes  converge  and  the  pupils  contract.  These 
are  not,  however,  essential  in  the  sense  in  which  the  changes 
in  the  lens  are  ;  for,  as  before  stated,  in  the  absence  of  the  lens 
distinct  vision  is  quite  impossible. 


ALTERATIONS   IN   THE    SIZE   OF   THE   PUPIL. 

The  pupil  varies  in  size  according  as  the  iris  is  in  a  greater 
or  less  degree  active.  All  observers  are  agreed  that  the  circu- 
lar fibers  around  the  pupillary  margin  are  muscular,  forming 
the  so-called  sphincter  of  the  iris;  but  great  differences  of  opin- 
ion still  exist  in  regard  to  the  radiating  fibers.  It  is  thought 
by  many  that  all  the  changes  in  the  iris  may  be  explained  by 
the  elasticity  of  its  structure  without  assuming  the  existence 
of  muscular  fibers  other  than  those  of  the  sphincter  ;  thus  a 
contraction  of  the  latter  would  result  in  diminution  of  the  pu- 


53J: 


COMPARATIVE   PHYSIOLOGY. 


pillary  aperture,  its  relaxation  to  an  enlargement,  provided  the 
rest  of  the  iris  were  highly  elastic. 

The  conclusions  in  regard  to  the  innervation  of  the  iris  rest 
largely  upon  the  results  of  certain  experiments  which  we  shall 

Brain  above  medulla 


Optic  centre 


Retina 


Sympathetic  nerve  to 
radiating  fibres 


Spinal  dilator  centre  — 


Fig.  383. — Diagram  to  illustrate  innervation  of  the  iris.    Dotted  lines  indicate  general 
functional  connection  (correlation).    Course  of  impulses  indicated  by  arrows. 

now  hriefly  detail  :  1.  When  the  third  nerve  is  divided,  stimu- 
lation of  the  optic  nerve  (or  retina)  does  not  cause  contraction 
of  the  pupil  as  usual.  2.  When  the  optic  nerve  is  divided,  light 
no  longer  causes  a  contraction  of  the  pupil,  though  stimulation 
of  the  third  nerve  or  its  center  in  the  anterior  portion  of  the 
floor  of  the  aqueduct  of  Sylvius  does  hring  about  this  result. 
3.  Section  of  the  cervical  sympathetic  is  followed  by  contrac- 


VISION.  535 

tion  and  stimulation  of  its  peripheral  end  by  dilatation  of  the 
pupil. 

From  such  experiments  it  has  been  concluded  that — 1.  The 
optic  is  the  afferent  nerve  and  the  third  nerve  the  efferent  nerve 
concerned  in  the  contraction  of  the  pupil  ;  and  that  the  center 
in  the  brain  is  situated  as  indicated  above,  so  that  the  act  is  or- 
dinarily a  reflex.  2.  That  the  cervical  sympathetic  is  the  path 
of  the  efferent  impulses  regulating  the  action  of  the  radiating 
fibers  of  the  iris. 

Its  center  has  been  located  near  that  for  the  contraction  of 
the  pupil,  and  it  may  be  assumed  to  exert  a  tonic  action  over 
the  iris  comparable  to  that  of  the  vaso-motor  center  over  the 
blood-vessels. 

The  impulses  may  be  traced  through  the  cervical  sympa- 
thetic and  its  ganglia  back  to  the  first  thoracic  ganglion,  and 
thence  to  the  spinal  cord  and  brain.  There  may  be  subsidiary 
centers  in  the  cervical  spinal  cord. 

It  is  to  be  remembered  that,  although  the  dilating  center  is 
automatic  in  action,  it  may  also  act  reflexly,  or  be  modified  by 
unusual  afferent  impulses — as,  e.  g.,  the  strong  stimulation  of 
any  sensory  nerve  which  causes  enlargement  of  the  pupil 
through  inhibition  of  the  center.  To  render  the  paths  of 
impulses  affecting  the  iris  somewhat  clearer,  it  is  well  to  bear 
in  mind  the  nervous  supply  of  the  part  :  1.  The  third  nerve, 
through  the  ciliary  (ophthalmic,  lenticular)  ganglion,  supplies 
short  ciliary  nerves  to  the  iris,  ciliary  muscle,  and  choroid.  2. 
The  cervical  sympathetic  reaches  the  iris  chiefly  through  the 
long  ciliary  nerves  and  the  ophthalmic  division  of  the  fifth. 
3.  There  are  sensory  fibers  from  the  fifth  nerve ;  and.  according 
to  some  observers,  also  dilating  Abel's  from  this  nerve  inde- 
pendent of  the  sympathetic,  as  well  as  those  that  may  reach 
the  eye  by  the  long  ciliary  nerves  without  entering  the  ciliarv 
ganglion.  4.  The  centers  from  which  both  the  contracting  and 
dilating  impulses  proceed  are  situated  near  to  each  other  in 
the  floor  of  the  aqueduct  of  Sylvius.  It  is  of  practical  im- 
portance to  remember  the  various  circumstances  under  which 
the  pupil  contracts  and  dilates. 

Contraction  (Myosis).—l.  Access  of  strong  light  to  the 
retina.  2.  Associated  contraction  on  accommodation  for  near 
objects.  3.  Similar  associated  contraction  when  the  visual  axes 
converge,  as  in  accommodation  for  near  objects.  4.  Reflex 
stimulation  of  afferent  nerves,  as  the  nasal  or  ophthalmic  divis- 


536  COMPARATIVE  PHYSIOLOGY. 

ion  of  the  fifth  nerve.  5.  During  sleep.  6.  Upon  stimulation 
of  the  optic  or  the  third  nerve,  and  the  corpora  quadrigemina 
or  adjacent  parts  of  the  brain.  7.  Under  the  effects  of  certain 
drugs,  as  physostignhn,  morphia,  etc. 

Dilatation  {Mydriasis). — 1.  In  darkness.  2.  On  stimulation 
of  the  cervical  sympathetic.     3.  During  asphyxia  or  dyspnoea. 

4.  By  painful  sensations  from  irritation  of  peripheral  parts. 

5.  From  the  action  of  certain  drugs,  as  atropin,  etc.  The 
student  may  impress  most  of  these  facts  upon  his  mil  id  by 
making  the  necessary  observations,  which  can  be  readily  done. 

Pathological. — As  showing  the  importance  of  such  connec- 
tions, we  may  instance  the  fact  that,  in  certain  forms  of  nervous 
disease  (e.  g.,  locomotor  ataxia),  the  pupil  contracts  when  the 
eye  is  accommodated  to  near  objects,  but  not  to  light  (the 
Argyll-Robertson  pupil).  In  other  cases,  owing  to  brain-dis- 
ease, the  pupils  may  be  constantly  dilated  or  the  reverse  ;  or 
one  may  be  dilated  and  the  other  contracted. 

Comparative. — The  iris  varies  in  color  in  different  groups  of 
animals,  and  even  in  individuals  of  the  same  group  ;  while  the 
color  in  early  life  is  not  always  the  permanent  one. 

In  shape  the  pupil  is  elliptical  in  solipeds  and  most  rumi- 
nants. In  the  pig  and  dog  it  is  circular,  as  also  in  the  cat 
when  dilated ;  but  when  greatly  contracted  in  the  latter  animal, 
it  may  become  a  mere  perpendicular  slit. 

The  iris  is  covered  posteriorly  with  a  layer  of  pigment  (uvea), 
portions  of  which  may  project  through  the  pupil  into  the  an- 
terior chamber,  and  constitute  the  "  sootballs  "  (corpora  nigra) 
well  seen  in  horses,  and  very  suggestive  of  inflammatory 
growths,  though,  of  course,  perfectly  normal. 

OPTICAL   IMPERFECTIONS  OF   THE   EYE. 

Anomalies  of  Refraction.— 1.  We  may  speak  of  an  eye  in 
which  the  refractive  power  is  such  that,  under  the  limitations 
referred  to  before  (page  531),  images  are  focused  on  the  retina, 
as  the  emmetropic  eye.  The  latter  is  illustrated  by  Fig.  384. 
In  the  upper  figure,  in  which  the  eye  is  represented  as  passive 
(negatively  accommodated),  parallel  rays—  i.  e.,  rays  from  ob- 
jects distant  more  than  about  seventy  yards  (according  to  some 
writers  much  less) — are  focused  on  the  retina  ;  but  those  from 
objects  near  at  hand,  the  rays  from  which  are  divergent,  are 
focused  behind  the  retina.     In  the  lower  figure  the  lens  is  rep- 


VISION. 


537 


resented  as  more  bulging,  from  accommodation,  as  such  diver- 
gent rays  are  properly  focused. 

2.  In  the  myopic  (near-sighted)  eye  the  parallel  rays  cross 
within  the  vitreous  humor,  and  diffusion-circles  being  formed 
on  the  retina,  the  image  of  the  object  is  necessarily  blurred, 


Vs^r. 


Fig.  384. — Diagrams  to  illustrate  conditions  of  refraction  in  normal  eye  when  unac- 
commodated (passive,  or  nearly  accommodated),  and  when  accommodated  for 
"near"  objects  (after  Landois). 

so  that  an  object  must,  in  the  case  of  such  an  eye,  be  brought 
unusually  near,  in  order  to  be  seen  distinctly — i.  e.,  the  near 
■point  is  abnormally  near  and  the  far  point  also,  for  parallel 


Fig.  385.— Anomalies  of  refraction  in  a  myopic  eye  (after  Landois). 

rays  can  not  be  focused  ;  so  that  objects  must  be  near  enough 
for  the  rays  from  them  that  enter  the  eye  to  be  divergent. 

The  myopic  eye  is  usually  a  long  eye,  and,  though  the 
mechanism  of  accommodation  may  be  normal,  it  is  not  so 
usually,  the  ciliary  muscle  being  frequently  defective  in  some 
of  its  fibers,  which  may  be  either  hypertrophied  or  atrophied,  or 
with  some  affected  one  way  and  others  in  the  opposite.     More- 


538  COMPARATIVE   PHYSIOLOGY. 

over,  there  is  also  generally,  in  bad  cases,  "  spasm  of  accommo- 
dation" (i.  e.,  of  the  ciliary  muscle),  with  increased  ocular  ten- 
sion, etc.  The  remedies  are,  rest  of  the  accommodation  mechan- 
ism and  the  use  of  concave  glasses. 

3.  The  opposite  defect  is  hypermetropia.  The  hypermetropic 
eye  (Fig.  386),  being  too  short,  parallel  rays  are  focused  be- 
hind the  retina  ;  hence  no  distinct  image  of  distant  objects  can 


Fig.  386.— Anomalies  of  refraction  in  the  hypermetropic  eye  (after  Landois). 

be  formed,  and  they  can  only  be  seen  clearly  by  the  use  of  con- 
vex glasses,  except  by  the  strongest  efforts  at  accommodation. 
When  the  eye  is  passive,  no  objects  are  seen  distinctly  beyond 
a  certain  distance — i.e.,  the  near  point  is  abnormally  distant 
(eight  to  eighty  inches).  The  defect  is  to  be  remedied  by  the 
use  of  convex  glasses. 

4.  Presbyopia,  resulting  from  the  presbyopic  eye  of  the  old,  is 
owing  to  defective  focusing  power,  partly  from  diminished 
elasticity  (and  hence  flattening)  of  the  lens,  but  chiefly,  proba- 
bly, to  weakness  of  the  ciliary  muscle,  so  that  the  changes 
required  in  the  shape  of  the  lens,  that  near  objects  may  be  dis- 
tinctly seen,  can  not  be  made.  The  obvious  remedy  is  to  aid 
the  weakened  refractive  power  by  convex  glasses.  It  is  practi- 
cally important  to  bear  in  mind  that,  as  soon  as  any  of  these 
defects  in  refractive  power  (though  the  same  remark  applies  to 
all  ocular  abnormalities)  are  recognized,  the  remedy  should  be 
at  once  applied,  otherwise  complications  that  may  be  to  a  large 
extent  irremediable  may  ensue. 

VISUAL    SENSATIONS. 

We  have  thus  far  considered  merely  what  takes  place  in  the 
eye  itself  or  the  physical  causes  of  vision,  without  reference  to 
those  nervous  changes  which  are  essential  to  the  perception  of 


VISION. 


539 


an  object.  It  is  true  that  an  image  of  the  object  is  formed  on 
the  retina,  but  it  would  be  a  very  crude  conception  of  nervous 
processes,  indeed,  to  assume  that  anything  resembling  that 
image  were  formed  on  the  cells  of  the  brain,  not  to  speak  of 
the  superposition  of  images  inconsistent  with  that  clear  mem- 
ory of  objects  we  retain.  Before  an  object  is  "  seen,"  not  only 
must  there  be  a  clear  image  formed  on  the  retina,  but  impulses 
generated  in  that  nerve  expansion  must  be  conducted  to  the 
brain,  and  rouse  in  certain  cells  there  peculiar  molecular  condi- 
tions, upon  which  the  perception  finally  depends. 

For  the  sake  of  clearness,  we  may  speak  of  the  changes 


Fig.  387.  Fig.  388. 

Fig.  387.— Vertical  section  of  retina  (after  H.  Mflller).  1,  layer  of  rods  and  cones;  2 
rods;  8,  cones;  4,  5,  6,  external  granule  layer;  7,  interna!  granule  layer;  !),  10  fine- 
ly granular  gray  layer;  11,  layer  of  nerve-cells;  1:2,14,  fibers  of  optic  nerve;  13 
membrana  hmitans. 

Fio.  388.— Connection  of  rods  and  cones  of  retina  with  nervous  elements  (after  Sap- 
|>ey)  1,2,3,  rods  and  cones  seen  from  in  front;  4,  5,0,  side  view.  The  rest  will 
be  clear  from  the  preceding  figure. 


540 


COMPARATIVE   PHYSIOLOGY. 


effected  in  the  retina  as  sensory  impressions  or  impulses,  which, 
when  completed  by  corresponding  changes  in  the  brain,  develop 
into  sensations,  which  are  represented  psychically  by  percep- 
tions ;  hence,  though  all  these  have  a  natural  connection,  they 
may  for  the  moment  be  considered  separately.  It  is  as  yet 
beyond  our  power  to  explain  how  they  are  related  to  each 
other  except  in  the  most  general  way,  and  the  manner  in  which 
a  mental  perception  grows  out  of  a  physical  alteration  in  the 
molecules  of  the  brain  is  at  present  entirely  beyond  human 
comprehension . 

Affections  of  the  Eetina.— There  is  no  doubt  that  the  fibers  of 
the  optic  nerves  can  not  of  themselves  be  directly  affected  by 
light.  This  may  be  experimentally  demonstrated  to  one's  self 
by  a  variety  of  methods,  of  which  the  following  is  readily  car- 
ried out :  Look  at  the  circle  (Fig.  389)  on  the  left  hand  with  the 


Fig.  389  (after  Bernstein). 


right  eye,  the  left  being  closed,  and,  with  the  page  about  twelve 
to  fifteen  inches  distant,  gradually  approximate  it  to  the  eye, 
when  suddenly  the  cross  will  disappear,  its  image  at  that  dis- 


j:<)j^ry:;:t;:l;r,!r';''^. ...;■■'.■'../■;,  "":i"  ...j,    ■•■■.,■':■■'  ■■';:i,',\'ui/il,,!,";,/lj,i!1, 


m/m 


S-e- 


Fig.  390.—  Diagrammatic  section  of  macula  lutea  in  man  (after  Huxley),  a,  a,  pigment 
of  choroid;  b,  c,  b,  c,  rods  and  cones;  d,  d,  outer  granular  or  nuclear  layer;  /,/, 
inner  granular  layer;  g,  ij,  molecular  layer;  h,  h,  layer  of  nerve-cells;  i,  i,  fibers  of 
optic  nerve. 


VISION.  511 

tance  having  fallen  on  the  blind-spot,  or  the  point  of  entrance 
of  the  optic  nerve. 

It  remains,  then,  to  determine  what  part  of  the  retina  is 
affected  by  light.  The  evidence  that  it  is  the  layers  of  rods  and 
cones  is  convincing.  It  has  been  shown  that  parts  of  the  retina 
itself  internal  to  these  layers  cast  perceptible,  shadows,  the  con- 
clusion that  the  rods  and  cones  are  the  essential  parts  of  the 
sensory  organ  would  be  inevitable. 

The  Laws  of  Retinal  Stimulation. — It  may  be  noticed  that, 
when  a  circular  saw  in  a  mill  is  rotated  with  extreme  rapidity, 
it  seems  to  be  at  rest. 

If  a  stick  on  fire  at  one  end  be  rapidly  moved  about,  there 
seems  to  be  a  continuous  fiery  circle. 

If  a  top  painted  in  sections  with  various  colors  be  spun,  the 
different  colors  can  not  be  distinguished,  but  there  is  a  color 
resulting  from  the  blending  of  the  sensations  from  them  all, 
which  will  be  white  if  the  spectral  colors  be  employed. 

When,  on  a  dark  night,  a  moving  animal  is  illuminated  by 
a  flash  of  lightning,  it  seems  to  be  at  rest,  though  the  attitude  is 
one  we  know  to  be  appropriate  for  it  during  locomotion. 

It  becomes  necessary  to  explain  these  and  similar  phe- 
nomena. Another  observation  or  two  will  furnish  the  data  for 
the  solution. 

If  on  awakening  in  the  morning,  when  the  eyes  have  been 
well  rested  and  the  retina  is  therefore  not  so  readily  fatigued, 
one  looks  at  the  window  for  a  few  seconds  and  then  closes  the 
eyes,  he  may  perceive  that  the  picture  still  remains  visible  as 
a  positive  after-image ;  while,  if  a  light  be  gazed  upon  at  night 
and  the  eyes  suddenly  closed,  an  after-image  of  the  light  may 
be  observed. 

It  thus  appears,  then,  that  the  impression  or  sensation  out- 
lasts the  stimulus  in  these  cases,  and  this  is  the  explanation 
into  which  all  the  above-mentioned  facts  fit.  When  the  fiery 
point  passing  before  the  eyes  hi  the  case  of  the  fire-brand  stimu- 
lates the  same  parts  of  the  retina  more  frequently  than  is  con- 
sistent with  the  time  required  for  the  previous  impression  to 
fade,  there  is,  of  necessity,  a  continuous  sensation,  which  is  in- 
terpreted by  the  mind  as  referable  to  one  object.  In  like  man- 
ner, in  the  case  of  a  moving  object  seen  by  an  electric  flash,  the 
duration  of  the  latter  is  so  brief  that  the  object  illuminated  can 
not  make  any  appreciable  change  of  position  while  it  lasts;  a 
second   flash  would   show  au  alteration,  another  part  of  the 


542  COMPARATIVE   PHYSIOLOGY. 

retina  being  stimulated,  or  the  original  impression  having 
faded,  etc. 

In  the  case  of  a  top  or  (better  seen)  color-disk,  painted  into 
black  and  white  sectors,  it  may  be  observed  that  with  a  faint 
light  the  different  colors  cease  to  appear  distinct  with  a  slower 
rotation  than  when  a  bright  light  is  used.  The  variation  is 
between  about  TV  and  -5V  of  a  second,  according  to  the  intensity 
of  the  light  used.  Fusion  is  also  readier  with  some  colors  than 
others. 

It  is  a  remarkable  fact  that  one  can  distinguish  as  readily 
between  the  quantity  of  light  emanating  from  10  and  11  candles 
as  between  100  and  110. 

The  Visual  Angle. — If  two  points  be  marked  out  with  ink  on 
a  sheet  of  white  paper,  so  close  together  that  they  can  be  just 
distinguished  as  two  at  the  distance  of  12  to  20  inches,  then  on 
removing  them  a  little  farther  away  they  seem  to  merge  into 
one. 

The  principle  involved  may  be  stated  thus :  When  the  dis- 
tance between  two  points  is  such  that  they  subtend  a  less  visual 
angle  than  60  seconds,  they  cease  to  be  distinguished  as  two. 
Fig.  391  illustrates  the  visual  angle.  It  will  be  noticed  that  a 
larger  object  at  a  greater  distance  subtends  the  same  visual 
angle  as  a  smaller  one  much  nearer.     The  size  of  the  retinal 


Fig.  391.— The  visual  angle.    The  object  at  A"  appears  no  larger  than  the  one  at  .4 
(Le  Conte). 

image  corresponding  to  60  seconds  is  "004  mm.  (4  n),  and  this 
is  about  the  diameter  of  a  single  rod  or  cone.  It  is  not,  how- 
ever, true  that  when  two  cones  are  stimulated  two  objects  are 
inferred  to  exist  in  every  case  by  the  mind ;  for  the  retina  va- 
ries in  different  parts  very  greatly  in  general  sensibility  and  in 
sensibility  to  color. 

It  is  noticeable  that  visual  discriminative  power  can  be 
greatly  improved  by  culture,  a  remark  which  applies  especially 
to  colors.  It  seems  altogether  probable  that  the  change  is  cen- 
tral in  the  nerve-cells  of  the  part  or  parts  of  the  brain  con- 


VISION.  543 

cerned,  especially  of  the  cortical  region,  where  the  cell  processes 
involved  in  vision  are  finally  completed. 

Color- Vision, — As  we  are  aware  by  experience  the  range  and 
accuracy  of  color  perception  in  man  is  very  great,  though  vari- 
able for  different  persons,  a  good  deal  being  dependent  on  culti- 
vation. However,  there  are  also  pronounced  natural  differ- 
ences, some  individuals  being  unable  to  differentiate  between 
certain  primary  colors  as  red  and  green,  and  so  are  "  color- 
blind." It  is  of  course  difficult  to  determine  in  how  far  the 
lower  animals  can  discriminate  between  colors ;  but  in  certain 
groups,  as  the  birds,  it  would  seem  to  be  reasonable  to  conclude 
that  their  color-perceptions  are  highly  developed. 

It  is  further  probable  that  in  this  group,  and  possibly  some 
others  with  the  eyes  placed  more  in  the  lateral  than  the  anterior 
portion  of  the  head,  the  retinal  area  for  the  most  distinct  vision, 
including  that  for  colors  is  larger  than  in  man,  at  all  events. 

PSYCHOLOGICAL  ASPECTS  OF  VISION. 

It  is  impossible  to  ignore  entirely,  in  treating  of  the  physi- 
ology of  the  senses,  the  mind,  or  perceiving  ego. 

By  virtue  of  our  mental  constitution,  we  refer  what  we  "  see  " 
to  the  external  world,  though  it  is  plain  that  all  that  we  per- 
ceive is  made  up  of  certain  sensations. 

We  recognize  the  "  visual  field  "  as  that  part  of  the  outer 
world  within  which  alone  our  vision  can  act  at  any  one  time ; 
and  this  is,  of  course,  smaller  for  one  than  for  both  eyes. 

If  one  takes  a  large  sheet  of  paper  and  marks  on  its  center 
a  spot  on  which  one  or  both  eyes  are  fixed,  by  moving  a  point 
up  or  down,  to  the  right  or  the  left,  he  may  ascertain  the  limits 
of  the  visual  field  for  a  plane  surface.  The  visual  field  for  both 
eyes  measures  about  180°  in  the  horizontal  meridian ;  for  one 
eye  about  145°;  and  in  the  vertical  meridian  100°. 

After-images,  etc. — Positive  after-images  have  already  been 
referred  to ;  but  an  entirely  different  result,  owing  to  exhaus- 
tion of  the  retina,  may  follow  when  the  eye  is  turned  from  the 
object.  If,  after  gazing  some  seconds  at  the  sun,  one  turns 
away  or  merely  closes  the  eyes,  he  may  see  black  suns.  In 
like  manner,  when  one  turns  to  a  gray  surface  after  keeping 
the  eyes  fixed  on  a  black  spot  on  a  white  ground,  he  will  see  a 
light  spot.  Such  are  termed  negative  after-images,  and  these 
may  themselves  be  colored,  as  when  one  turns  from  a  red  to  a 


544 


COMPARATIVE   PHYSIOLOGY. 


white  surface  and  sees  the  latter  green, 
siclered  as  the  results  of  exhaustion. 


They  may  be  con- 


CO-ORBINATION   OF   THE   TWO   EYES    IN  VISION. 

As  a  matter  of  fact,  we  are  aware  that  an  object  may  be 
seen  as  one  either  with  a  single  eye  or  with  both.  For  bi- 
nocular vision  it  may  be  shown  that 
the  images  formed  on  the  two  retinas 
must  fall  invariably  on  corresponding 
points. 

The  position  of  the  latter  may  be 
gathered  from  Fig.  392.  It  will  be  no- 
ticed that  the  malar  side  of  one  eye 
corresponds  to  the  nasal  side  of  the 
other,  though  upper  always  answers  to 
upper  and  lower  to  lower.  This  may 
also  be  made  evident  if  two  saucers 
(representing  the  fundus  of  each  eye) 
be  laid  over  each  other  and  marked  off, 
as  in  the  figure. 

That  such   corresponding  points  do 

Fig.  392.— Diagram  to  illus-   actually  exist  maybe  shown  by  turniug 

(afterC  Foster)11  z?j?°leit   one  eye  so  that  the  image  shall  not 

pltttteo?,ee8;eye''iree-   &11,  *>  indicated  in  the  figure.     Only 

eponding  to  ax,  bu  cx,  m  now  au(j  then,  however,  is  a  person  to 

the  other.     The  lower  fig-  '  A 

ures  are  projections  of  the  be  found  wTho  can  voluntarily  acco.n- 

tffitaft (l?ey"S  Itmay be  plish  this,  but  it  occurs  in  all  kinds  of 

side'oTone'reti^  fovrl  natural  or  induced  squint,  as  inalcohol- 

■    sponds  to  the  nasal  side  of  {sm  owing1  to  partial  paralysis  of  some 

tliG  other  —Z. 

of  the  ocular  muscles.  We  are  thus 
naturally  led  to  consider  the  action  of  these  muscles. 

Ocular  Movements. — Upon  observing  the  movements  of  an 
individual's  eyes,  the  head  being  kept  stationary,  it  may  be 
noticed  that  (1)  both  eyes  may  converge ;  (2)  one  diverge  and 
the  other  turn  inward ;  (3)  both  move  upward  or  downward ; 
(4)  these  movements  may  be  accompanied  by  a  certain  degree 
of  rotation  of  the  eyeball. 

The  eye  can  not  be  rotated  around  a  horizontal  axis  without 
combining  this  movement  with  others.  To  accomplish  the 
above  movements  it  is  obvious  that  certain  muscles  of  the  six 
with  which  the  eye  is  provided  must  work  in  harmony,  both  as 


VISION. 


545 


to  the  direction  and  degree  of  the  movement — i.  e.,  the  move- 
ments of  the  eyes  are  affected  hy  very  nice  muscular  co-ordina- 
tions. 


Fig.  303.— View  of  the  two  eyes  and  related  parts  (after  Helmholtz.) 


Fig-.  394  is  meant  to  illustrate  diagrammatically  the  move- 
ments of  the  eyeball. 

While  the  several  recti  muscles  elevate  or  depress  the  eye, 
and  turn  it  inward  or  outward,  and  the  oblique  muscles  rotate 
it,  the  movements  produced  by  the  superior  and  inferior  recti 
always  corrected  by  the  assistance  of  the  oblique  muscles,  since 
the  former  tend  of  themselves  to  turn  the  eye  somewhat  in- 
ward. In  like  manner  the  oblique  muscles  are  corrected  by 
the  recti.  The  following  tabular  statement  will  expi*ess  the 
conditions  of  muscular  contraction  for  the  various  movements 
of  the  eye  in  man : 


Straight 
move- 
ments;. 


Elevation Rectus  superior  and  obliquus  in- 
ferior. 
Depression Rectus  inferior  and  obliquus  su- 
perior. 
|  Adduction  to  nasal  side. .  .Rectus  interims, 
i.  Adduction  to  malar  side. .  .  Rectus  externus. 
35 


546 


COMPARATIVE  PHYSIOLOGY. 


Oblique 
move-    - 
ments. 


r  Elevation  with  adduction. .  Rectus    superior    and    internus, 

with  obliquus  inferior. 
Depression  with  adduction.Reetus    inferior    and     internus, 

with  obliquus  superior. 
Elevation  with  abduction. .  Rectus    superior    and   externus, 

with  obliquus  inferior. 
Depression  with  abduction.Rectus    inferior    and    externus, 

with  obliquus  superior. 

"What  is  the  nervous  mechanism  by  which  these  "  associ- 
ated "  movements  of  the  eyes  are  accomplished  ?     It  has  been 

found,  experimental- 
ly, that  when  different 
parts  of  the  corpo- 
ra quaclrigemina  are 
stimulated,  certain 
movements  of»the  eyes 
follow.  Thus  stimu- 
lation of  the  right  side 
of  the  nates  leads  to 
movements  of  both 
eyes  to  the  left,  and 
the  reverse  when  the 
opposite  side  is  stimu- 
lated ;  also,  stimula- 
tion in  the  middle  line 
causes  convergence 
and  downward  move- 
ment, etc.,  with  the 
corresponding  move- 
ments of  the  iris. 
Since  section  of  the 
nates  in  the  middle 
line  leads  to  move- 
ments confined  to  the 
eye  of  the  same  side, 
the  center  would  ap- 
pear to  be  double. 
However,  it  may  be  that  the  cells  actually  concerned  do  not  lie 
in  the  corpora  quadrigemina,  but  below  or  outside  of  them.  The 
localization  is  as  yet  incomplete.  In  many  groups  of  animals, 
including  the  solipeds,  ruminants,  and  Carnivoi^a,  there  is  a 
posterior  rectus  or  retractor  oculi  by  which  the  eye  may  be 


Fig.  304.— Diagram  intended  to  illustrate  action  of 
extrinsic  ocular  muscles  (after  Pick).  The  heavy 
lines  represent  the  muscles  of  the  eyeball,  and  the 
fine  lines  the  axes  of  movement. 


VISION. 


547 


Fig.  395. — Diagrammatic  section  of  the  eye  of  the  horse  (Chauvean).  a,  optic  nerve; 
b.  sclerotic  coat;  c,  choroid;  d,  retina;  e,  cornea;  /,  iris;  g,  h,  ciliary  ligament  and 
processes  of  choroid  represented  as  separated  from  it,  the  better  to  define  its  lim- 
its; i,  insertion  of  ciliary  processes  on  crystalline  lens;  j,  crystalline  lens;  k\  lens 
capsule;  /,  vitreous  body;  m,m,  anterior  and  posterior  chambers;  o.  theoretical 
indication  of  aqueous  humor;  p,  p.  tarsi  (eyelids);  q,  q,  fibrous  membrane  of  eye- 
lids; r,  elevator  muscle  of  upper  eyelid;  s.  s,  orbicularis  muscle  of  lids;  t,  t,  skin 
of  eyelids;  u,  conjunctiva;  ?>,  epidermic  layer  of  the  latter  covering  cornea;  x, 
posterior  rectus  muscle;  y,  superior  rectus;  z,  inferior  rectus;  w,  fibrous  sheath  of 
orbit  (orbital  membrane). 

drawn  inward  and  thus  protected  the  more  effectually  against 
blows  and  obstacles.  It  seenis  to  be  of  special  importance  in 
animals  that  feed  with  the  head 
down  for  long-  periods,  as  in  the 
ruminants,  in  which  class  it  is 
most  highly  developed. 

The  macula  lutea  is  believed 
to  exist  only  in  man,  the  quadra- 
man  a.  and  certain  of  the  lizard 
tribe — i.  e.,  in  animals  in  which 
the  axes  of  the  eyeballs  are  parallel 
to  each  other.  Nevertheless,  there 
is  no  reason  to  doubt  that  the  cen- 
tral part  of  the  retina  is  more  sensitive  than  the  periphery  or 
that  there  is  a  central  retinal  zone  for  distinct  vision  in  all 
vertebrates,  though  not  so  limited  in  all  cases  as  in  man. 


Fig.  896. — Diagram  to  illustrate  de- 
cussation of  fibers  in  the  optic 
commissure  of  man  (after  Flint). 


548  COMPARATIVE  PHYSIOLOGY. 

Estimation  of  the  Size  and  Distance  of  Objects.— The  pro- 
cesses by  which  we  form  a  judgment  of  the  size  and  distance  of 
objects  are  closely  related. 

As  we  have  already  shown  (page  542),  the  visual  angle  varies 
both  with  the  size  and  the  distance  of  an  object.  Knowing 
that  two  objects  are  at  the  same  distance  from  the  eye,  we  esti- 
mate that  the  one  is  larger  than  the  other  when  the  image  one 
forms  on  the  retina  is  larger,  or  when  the  visual  angle  it  sub- 
tends is  greater  than  in  the  other  case,  and  conversely.  Thus, 
knowing  that  two  persons  are  at  the  distance  of  half  a  mile 
away,  if  one  is  judged  by  us  to  be  smaller  than  the  other,  it 
will  be  because  the  retinal  image  corresponding  to  the  object 
is  smaller,  other  things  being  equal.  But  the  subject  is  more 
complex  than  might  be  inferred  from  these  statements. 

Objects  of  a  certain  color  seem  nearer  than  others ;  also  those 
that  are  brighter,  as  in  the  case  of  mountains  on  a  clear  day. 
And  not  only  do  all  the  qualities  of  the  image  itself  enter  as 
data  into  the  construction  of  the  judgment,  but  numerous  mus- 
cular sensations.  The  eyes  accommodating  and  converging  for 
near  objects,  from  the  law  of  association,  give  rise  to  the  idea  of 
nearness,  for  habitually  such  takes  place  when  near  objects  are 
viewed,  so  that  the  subject  becomes  very  complex.  That  we 
judge  imperfectly  of  the  position  of  an  object  with  but  one  eye 
is  realized  on  attempting  to  stick  a  pin  into  a  certain  small  spot, 
thread  a  needle,  cork  a  small  bottle,  etc.,  when  one  eye  is  closed. 

Solidity. — By  the  use  of  one  eye  alone  we  can  form  an  idea 
of  the  shape  of  a  solid  body  ;  though,  in  the  case  of  such  as  are 
very  complex,  this  process  is  felt  to  be  both  laborious  and  im- 
perfect. 

From  the  limited  nature  of  the  visual  field  for  distinct 
vision,  it  follows  that  we  can  not  with  one  eye  see  equally  dis- 
tinctly all  the  parts  of  a  solid  that  is  turned  toward  us.  After 
a  little  practice  one  may  learn  to  define  for  himself  what  he 
actually  does  see. 

Such  a  figure  as  that  following  results  from  the  combina- 
tion, mentally,  of  two  others,  which  answer  to  the  images  fall- 
ing on  the  right  and  on  the  left  eye  respectively. 

In  order  that  such  fusion  shall  take  place,  the  respective 
images  must  fall  on  identical  (corresponding)  parts  of  the 
retina. 

As  is  well  known,  the  pictures  used  for  stereoscopes  give 
different  views  of  the  one  object,  as  represented  on  a  flat  sur- 


VISION. 


549 


face.  These  are  thrown  upon  corresponding  points  of  the  retina 
by  the  use  either  of  prisms  or  mirrors,  when  the  idea  of  solidity 
is  produced.  As  to  whether  movements  of  the  eyes  (conver- 
gence) are  necessary  for  stereoscopic  vision  is  disputed.     It  has 


d/ 


a 

Fig.  397. — Illustrates  binocular  vision.  If  the  truncated  pyramid,  P,  be  looked  at 
with  the  head  held  perpendicularly  over  the  figure,  the  image  formed  in  the  right 
eye  when  the  left  is  closed  is  figured  on  the  right,  and  that  seen  when  the  right 
eye  is  closed  is  represented  by  the  figure  in  the  middle.  No  superposition  of  these 
figures  will  give  P,  yet  by  a  pyschic  process  they  are  combined  into  P,  the  figure 
as  it  appears  to  both  eyes  (after  Bernstein). 

been  inferred,  from  the  fact  that  objects  appear  solid  during 
an  electric  flash,  the  duration  of  which  is  far  too  short  to  per- 
mit of  movements  of  the  ocular  muscles,  that  such  movements 
are  not  essential.  The  truth  seems  to  lie  midway  ;  for  while 
simple  figures  may  not  require  them,  the  more  complex  do,  or, 
at  all  events,  the  judgment  is  very  greatly  assisted  thereby.  It 
is  of  the  utmost  importance  to  bear  in  mind  that  all  visual 
judgments  are  the  result  of  many  processes,  in  which,  not  the 
sense  of  vision  alone,  but  others,  are  concerned;  and  the  mutual 
dependence  of  one  sense  on  another  is  great,  probably  beyond 
our  powers  to  estimate.  Reference  has  been  made  to  this  sub- 
ject previously. 


PROTECTIVE    MECHANISMS   OF   THE   EYE. 

The  eyelids  have  been  appropriately  compared  to  the  shut- 
ters of  a  window.  They  are,  however,  not  impervious  to  light, 
as  any  one  may  convince  himself  by  noticing  that  he  can  locate 
the  position  of  a  bright  light  with  the  eyes  shut;  also  that  a 
sensitive  person  (child)  will  turn  away  (reflexly)  from  a  light 
when  sleeping  if  it  be  suddenly  brought  near  the  head.  The 
Meibomian  glands,  a  modification  of  the  sebaceous,  secrete  an 
oily  substance  that  seems  to  protect  the  lids  against  the  lachry- 
mal fluid,  and  prevents  the  latter  running  over  their  edges  as 
oil  would  on  the  margins  of  a  vessel.     The  lachrymal  gland  is 


550 


COMPARATIVE  PHYSIOLOGY. 


not  in  structure  unlike  the  parotid,  the  secretion  of  which  its 
own  somewhat  resembles. 

The  saltness  of  the  tears,  owing  to  abundance  of  sodium 
chloride,  is  well  known  to  all.  The  nervous  mechanism  of  se- 
cretion of  tears  is  usually  reflex,  the  stimulus  coming  from  the 
action  of  the  air  against  the  eyeball  or  from  partial  desiccation 
owing  to  evaporation.  When  the  eyeball  itself,  or  the  nose,  is 
irritated,  the  afferent  nerves  are  the  branches  of  the  fifth,  to 
which  also  belong  the  efferent  nerves.  Tbe  latter  include  also 
the  cervical  sympathetic.  But  it  will,  of  course,  be  understood 
that  the  afferent  impulses  may  be  derived  through  a  large  num- 
ber of  nerves,  and  that  the  secreting  center  may  be  acted  upon 
directly  by  the  cerebrum  (emotions).  The  excess  of  lachrymal 
secretion  is  carried  away  by  the  nasal  duct  into  which  tbe  lach- 
rymal canals  empty.  While  it  is  well  known  that  closure  of  the 
lids  by  the  orbicularis  muscle  favors  the  removal  of  the  fluid,  the 
method  by  which  the  latter  is  accomplished  is  not  agreed  upon. 
Some  believe  that  the  closure  of  the  lids  forces  the  fluid  on 
through  the  tubes,  when  they  suck  in  a  fresh  quantity ;  others  that 
the  orbicularis  drives  the  fluid  directly  through  the  tubes,  kept 
open  by  muscular  arrangements  ;  and  there  are  several  other 
divergent  opinions.  The  prevention  of  winking  leads  to  irrita- 
tion  of  the  eye,  which  may  assume  a  serious  character,  so  that 
the  obvious  use  of  the  secretion  of  tears 
is  to  keep  the  eye  both  moist  and  clean. 
Though  rudimentary  in  man,  there 
is  in  all  our  domestic  animals  a  third 
eyelid  {membrana  nictitans)  which 
may  be  made  to  sweep  over  the  eye 
and  thus  cleanse  it.  It  is  especially 
well  developed  in  those  groups  of 
mammals  that  can  not  derive  assist- 
ance in  wiping  the  eyes  from  their  fore- 
limbs,  hence  is  found  in  perfection  in 
solipeds  and  ruminants.  It  is  made  up 
of  a  fibro  -  cartilage,  prismatic  at  its 
base,  and  thus  anteriorly  where  it  is 
covered  by  the  conjunctiva.  It  is  most 
attached  at  the  inner  can  thus  of  the 
eye,  from  which  region  it  can  spread 
over  the  whole  globe  anteriorly.  The  fibro-cartilage  is  con- 
tinued backward  by  a  fatty  cushion  which  is  loosely  attached 


Fig.  398.— Lachrymal  canals, 
lachrymal  sac,  and  nasal 
canal  in  man  opened  from 
the  front  (after  Sappey.). 


VISION.  551 

to  all  the  ocular  muscles.  When  the  globe  of  the  eye  is  with- 
drawn by  its  muscles,  the  third  eyelid  is  pushed  out  in  a  me- 
chanical way  with  little  or  no  direct  assistance  from  muscles. 

In  this  connection  may  also  be  mentioned  the  gland  of 
Harder,  a  yellowish  red  glandular  structure  situated  about  the 
middle  of  the  outer  surface  of  the  third  eyelid,  which  furnishes 
a  thick  unctuous  secretion,  also  of  a  protective  character.  These 
structures  are  all  the  more  necessary,  as  in  few  animals  is  the 
globe  of  the  eye  so  well  protected  by  bony  walls  as  in  man. 

SPECIAL   CONSIDERATIONS* 

Comparative. — It  seems  to  be  established  that  some  animals 
devoid  of  eyes,  as  certain  myriopods,  are  able  to  perceive  the 
presence  of  light,  even  when  the  heat-rays  are  cut  off.  The  most 
rudimentary  beginning  of  a  visual  apparatus  appears  to  be  a 
mass  of  pigment  with  a  nerve  attached,  as  in  certain  worms  ; 
though  it  is  questionable  whether  mere  collections  of  pigment 
without  nerves  may  not  in  some  instances  represent  still  earlier 
rudiments  of  our  eyes. 

The  eye  of  the  fish  is  characterized  by  flatness  of  the  cornea ; 
spherical  form  of  the  lens,  the  anterior  surface  of  which  pro- 
jects far  beyond  the  pupillary  opening  ;  the  presence  of  a  pro- 
cess of  the  choroid  {processus  falciformis) ;  and  usually  the  ab- 
sence of  eyelids,  the  cornea  being  covered  with  transparent  skin. 

The  eye  of  the  bird,  in  some  respects  the  most  perfect  visual 
organ  known,  is  of  peculiar  shape  as  a  whole,  presenting  a  large 
posterior  surface  for  retinal  expansion  ;  a  very  convex  cornea, 
a  highly  developed  lens,  an  extremely  movable  iris  ;  eyelids 
and  a  nictitating  membrane  (third  eyelid),  which  may  be  made 
to  cover  the  whole  of  the  exposed  part  of  the  eye,  and  thus 
shield  screen -like  from  excess  of  light  ;  ossifications  of  the  scle- 
rotic ;  a  structure  which  is  a  peculiar  modification  of  the 
choroid,  of  which  it  is  a  sort  of  offshoot  and  like  it  very  vascular, 
answering  to  the  falciform  process  of  the  eye  of  the  fish  and  the 
reptile.  From  its  appearance  it  is  termed  the  pecten.  Birds,  on 
account  of  a  highly  developed  ciliary  muscle,  possess  wonderful 
powers  of  accommodation,  rendered  important  on  account  of 
their  rapid  mode  of  progression.  They  also  seem  to  be  able  to 
alter  the  size  of  the  pupil  at  will.  Their  iris  is  composed  of 
striped  muscular  fibers. 

A  layer  of  fibrous  tissue  outside  of  the  choroidal  epithelium 


552 


COMPARATIVE  PHYSIOLOGY. 


forms  the  tapetum.  It  is  most  pronounced  in  the  carnivora 
and  gives  the  glare  to  their  eyes  as  well  seen  in  the  cat  tribe  at 
night.  It  has  been  supposed  to  act  as  a  reflector  and  thus 
assist  in  vision  in  the  same  way  as  a  pair  of  carriage  lamps 
light  up  the  roadway. 

Evolution. — From  the  above  brief  account  of  the  eye  in  dif- 
ferent grades  of  animals,  it  will  ap- 
pear that  its  modifications  answer  to 
differences  in  the  environment. 

Adaptation  is  evident.  Darwin 
believes  this  to  have  been  effected 
partly  by  natural  selection — i.  e.,  the 
survival  of  the  animal  in  which  the 
form  of  eye  appeared  best  adapted  to 
its  needs — and  partly  by  the  use  or 
disuse  of  certain  parts. 

The  latter  is  illustrated  by  the 
blind  fishes,  insects,  etc.,  of  certain 
caves,  as  those  of  Kentucky;  and  it 
is  of  extreme  interest  to  note  that 
various  grades  of  transition  toward 
complete  blindness  are  observable, 
CM .ciliary  muscle.  Birds  have   according  to  the  degree  of  darkness 

usually  keen  vision,  great  pow-    ,  °  ° 

er  of  accommodation,  and  ex-   in  which  the  animal  lives,  whether 

treme  mobility  of  the  iris.  ,     ,,  ..,  .        ,,  , 

wholly  within  the  cave  or  where 
there  is  still  some  light.  A  crab  has  been  found  with  the  eye- 
stalk  still  present,  but  the  eye  itself  atrophied.  Again,  ani- 
mals that  burrow  seem  to  be  in  process  of  losing  their  eyes, 
through  inflammation  from  obvious  causes ;  and  some  of  them, 
as  the  moles,  have  the  eye  still  existing,  though  well-nigh  or 
wholly  covered  with  skin.  Internal  parasites  are  often  with- 
out eyes.  It  is  not  difficult  to  understand  how  one  bird  of  prey, 
with  eyes  superior  to  those  of  its  fellows,  would  gain  supremacy, 
and,  in  periods  of  scarcity,  survive  and  leave  offspring  when 
others  would  perish. 

It  is,  of  course,  impossible  to  trace  each  step  by  which  the 
vertebrate  eye  has  been  developed  from  more  rudimentary 
forms,  though  the  data  for  such  an  attempt  have  greatly  accu- 
mulated within  the  last  few  years;  and  it  is  not  to  be  forgotten 
that  even  the  vertebrate  eye  has  many  imperfections,  so  that 
no  doctrine  of  complete  adaptation,  according  to  the  argument 
from  design  as  usually  understood,  can  apply. 


Fig.  399.—  Eye  of  nocturnal  bird 
of  prey  (after  Wiedersheim). 
Co,  cornea;  L,  lens;  Rt,  retina; 
P,  pecten;  No,  optic  nerve;  Sc, 
ossification  of  sclerotic  coat; 


VISION. 


553 


It  is  of  great  importance  to  recognize  that  what  we  really 
see  depends  more  upon  the  brain  and  the  mind  than  the  eye. 
If  any  one  will  observe  how  frequent  are  his  incipient  errors 
of  vision  speedily  corrected,  he  will   realize  the   truth  of  the 


Brain  above 
medulla 


Centre  in  region  of- 
corp.  quadrigemina 


-Cortical  centre 


Centre  in  optic  thalamus 


etina 


Fig.  400. — Diagram  intended  to  illustrate  the  elaboration  of  visual  impulses,  beginning 
in  retina  and  culminating  in  the  cerebral  cortex.  Course  of  impulses  is  indicated 
by  arrows.  Knowledge  of  auditory  centers  is  not  yet  exact  enough  to  permit  of 
the  construction  of  a  diagram,  though  doubtless  eventually  the  central  processes 
will  be  localized  as  with  vision.  The  latter  remark  applies"  to  the  other  senses  to 
nearly  the  same  extent,  possibly  quite  as  much. 


above  remark.  Precisely  the  same  data  furnished  by  the  eye 
are  in  one  mind  wox\ked  up  in  virtue  of  past  experience  (edu- 
cation) into  an  elaborate  conception,  while  to  another  they  an- 
swer only  to  certain  vague  forms  and  colors.  And  herein  lies 
the  great  superiority  of  man's  vision  over  that  of  all  other 
animals. 

Within  the  limits  of  their  mental  vision  do  all  creatures  see. 
Man  has  not  the  keen  ocular  discriminating  power  of  the  hawk; 
he  can  neither  see  so  far  nor  so  clearly ;  nor  has  he  the  wide 
field  of  vision  of  the  gazelle;  but  he  has  the  mental  resource 
which  enables  him  to  make  more  out  of  the  materials  with 
which  his  eyes  furnish  him.  It  is  by  virtue  of  his  higher  cere- 
bral development  that  he  has  added  to  his  natural  eyes  others 


554  COMPARATIVE  PHYSIOLOGY. 

in  the  microscope  and  telescope,  which  none  of  Nature's  forms 
can  approach. 

Pathological. — There  may  be  ulceration  of  the  cornea,  in- 
flammation of  this  part,  or  various  other  disorders  which  lead 
to  opacity.  The  low  vitality  of  this  region,  probably  owing  to 
absence  of  blood-vessels,  is  evidenced  by  the  slowness  with 
which  small  ulcers  heal.  Opacity  of  the  lens  (cataract)  when 
complete  causes  blindness,  which  can  be  only  partially  reme- 
died by  removal  of  the  former.  Inflammations  of  any  part  of 
the  eye  are  serious,  from  possible  adhesions,  opacities,  etc.,  fol- 
lowing. Should  such  be  accompanied  by  great  excess  of  intra- 
ocular tension,  serious  damage  to  the  retina  may  result.  Of 
course,  atrophy  of  the  optic  nerve  (due  to  lesions  in  the  brain, 
etc.)  is  irremediable,  and  involves  blindness.  Inspection  of  the 
internal  parts  of  the  eye  (fundus  oculi)  often  reveals  the  first 
evidence  of  disease  in  remote  parts  as  the  kidneys. 

From  what  has  been  said  of  the  movements  of  the  two  eyes 
in  harmony,  etc. ,  the  student  might  be  led  to  infer  that  disease 
of  one  organ,  in  consequence  of  an  evident  close  connection  of 
the  nervous  mechanism  of  the  eyes,  would  be  likely  to  set  up 
a  corresponding  condition  in  the  other  unless  speedily  checked. 
Such  is  the  case,  and  is  at  once  instructive  and  of  great  practi- 
cal moment. 

Paralysis  of  the  various  ocular  muscles  leads  to  squinting, 
as  already  noticed. 

Brief  Synopsis  of  the  Physiology  of  Vision.— All  the  other 
parts  of  the  eye  may  be  said  to  exist  for  the  retina,  since  all  are 
.related  to  the  formation  of  a  distinct  image  on  this  nervous  ex- 
pansion. The  principal  refractive  body  is  the  crystalline  lens. 
The  iris  serves  to  regulate  the  quantity  of  light  admitted  to  the 
eye,  and  to  cut  off  too  divergent  rays.  In  order  that  objects  at 
different  distances  may  be  seen  distinctly,  the  lens  alters  in 
shape  in  response  to  the  actions  of  the  ciliary  muscle  on  the 
suspensory  ligament,  the  anterior  surface  becoming  more  con- 
vex. Accommodation  is  associated  with  convergence  of  the 
visual  axes  and  contraction  of  the  pupil.  The  latter  has  circular 
and  radiating  plain  muscular  fibers  (striped  in  birds,  that  seem 
to  be  able  to  alter  the  size  of  the  pupil  at  will),  governed  by  the 
third,  fifth,  and  sympathetic  nerves.  Contraction  of  the  pupil 
is  a  reflex  act,  the  nervous  center  lying  in  the  front  part  of  the 
Hoor  of  the  aqueduct  of  Sylvius,  while  the  action  of  the  other 
center  (near  this  one)  through  the  sympathetic  nerve  is  tonic. 


VISION.  555 

Accommodation  through  the  ciliary  muscle  is  governed  hy 
a  center  situated  in  the  hind  part  of  the  floor  of  the  third  ven- 
tricle near  the  anterior  bundles  of  the  third  nerve,  which  latter 
is  the  medium  of  the  change.  When  rays  of  light  are  focused 
anterior  to  the  retina,  the  eye  is  myopic ;  when  posterior  to  it, 
hypermetropic. 

The  presbyopic  eye  is  one  in  which  the  mechanism  of  accom- 
modation is  at  fault,  chiefly  the  ciliary  muscle.  The  point  of 
entrance  of  the  optic  nerve  (blind-spot)  is  insensible  to  light; 
and  visual  impulses  can  be  shown  to  originate  in  the  layers  of 
rods  and  cones,  probably  through  stimulation  from  chemical 
changes  effected  by  light  acting  on  the  retina.  The  sensation 
outlasts  the  stimulus ;  hence  positive  after-images  occur.  Nega- 
tive after-images  occur  in  consequence  of  excessive  stimulation 
and  exhaustion  of  the  retina,  or  disorder  of  the  chemical  pro- 
cesses that  excite  visual  impulses.  When  stimuli  succeed  one 
another  with  a  certain  degree  of  rapidity,  sensation  is  continu- 
ous. The  eye  can  distinguish  degrees  of  light  within  certain 
limits,  varying  by  about  -j^-g-  of  the  total. 

Objects  become  fused  or  are  seen  as  one  when  the  rays 
from  them  falling  on  the  retina  approximate  too  closely  on 
that  surface.  The  brain,  as  well  as  the  eye  itself,  is  concerned 
in  such  discriminations,  the  former  probably  more  than  the 
latter. 

The  macula  lutea,  and  especially  the  fovea  centralis,  are  in 
man  the  points  of  greatest  retinal  sensitiveness.  When  the 
images  of  objects  are  thrown  on  these  parts,  they  are  seen  with 
complete  distinctness;  and  it  is  to  effect  this  result  that  the 
movements  of  the  two  eyes  in  concert  take  place.  An  object  is 
seen  as  one  when  the  position  of  the  eyes  (visual  axes)  is  such 
that  the  images  formed  fall  on  corresponding  parts  of  the  retina. 
Binocular  vision  is  necessary  to  supply  the  sensory  data  for  the 
idea  of  solidity.  It  is  important  to  remember  that,  before  an  .ob- 
ject is  "  seen  "  at  all,  the  sensory  impressions  furnished  by  the 
retina  and  conveyed  inward  by  the  optic  nerve  are  elaborated  in 
the  brain  and  brought  under  the  cognizance  of  the  perceiving 
ego.  We  recognize  many  visual  illusions  and  imperfections 
of  various  kinds,  the  course  of  which  it  is  difficult  to  locate 
in  any  one  part  of  the  visual  tract,  such  as  are  referred  to 
"  irradiation,"  "  contrast,"  etc.  Tbere  may  also  be  visual  phe- 
nomena that  are  purely  subjective,  and  others  that  result  from 
suggestion   rather  than  any  definite   sensory  basis  of  retinal 


556  COMPARATIVE  PHYSIOLOGY. 

images.     Hence  what  one  sees  depends  on  his  state  of  mind  at 
the  time. 

This  applies  to  appreciation  of  size  and  distance  also,  though 
in  such  cases  we  have  the  visual  angle,  certain  muscular  move- 
ments (muscular  sense),  the  strain  of  accommodation  etc.,  as 
guides. 


HEARING. 


As  the  end  organ  oi  vision  is  protected  both  without  and 
within,  so  is  the  still  more  complicated  end-organ  of  the  sense 
of  hearing  more  perfectly  guarded  against  injury,  being  in- 
closed within  a  membranous  as  well  as  bony  covering  and  sur- 
rounded by  fluid,  which  must  shield  it  from  stimulation,  except 
through  this  medium. 

Hearing  proper,  as  distinguished  from  the  mere  recognition 
of  jars  to  the  tissues,  can,  in  fact,  only  be  attained  through  the 
impulses  conveyed  to  the  auditory  brain-centers,  as  originated 
in  the  end-organ  by  the  vibrations  of  the  fluid  with  which  it  is 
bathed. 

It  will  be  assumed  that  the  student  has  made  himself  famil- 
iar with  the  general  anatomy  of  the  ear.  The  essential  points 
in  regard  to  sound  are  considered  in  the  chapter  on  The 
Voice.  It  will  be  remembered  that  what  we  term  a  musical 
tone,  as  distinguished  from  a  noise,  is  characterized  by  the 
regularity  of  vibrations  of  the  air  that  reach  the  ear ;  and  that 
just  as  ethereal  vibrations  of  a  certain  wave-length  give  rise  to 
the  sensation  of  a  particular  color,  so  do  aerial  vibrations  of  a 
definite  wave-length  originate  a  certain  tone.  In  each  case 
must  we  take  into  account  a  physical  cause  for  the  physiological 
effect,  and  these  bear  a  very  exact  relationship  to  one  another. 

As  will  be  seen  later,  while  in  all  animals  that  have  a  well- 
defined  sense  of  hearing  the  process  is  essentially  such  as  we 
have  indicated  above,  the  means  leading  up  to  the  final  stimu- 
lation of  the  end-organ  are  very  various.  At  present  we  shall 
consider  the  acoustic  mechanism  in  mammals,  with  special  ref- 
erence to  man.  There  are  in  fact  three  sets  of  apparatus  :  (1) 
one  for  collecting  the  aerial  vibrations;  (2)  one  for  transmit- 
ting them ;  and  (3)  one  for  receiving  the  impression  through  a 
fluid  medium;  in  other  words,  an  external,  middle,  and  in- 
ternal ear. 


558 


COMPARATIVE   PHYSIOLOGY. 


The  external  ear  in  man  being-  practically  immovable,  owing 
to  the  feeble  development  of  its  muscles,  has,  as  compared  with 


;:ittl-n 


Ftg.  401.  —Section  through  auditory  organ  (after  Sappey).  1,  pinna;  2,  4,  5,  cavity  of 
concha,  external  and  auditory  meatus  with  opening  of  ceruminous  glands;  6, 
membrana  tympani;  7.  anterior  part  of  incus;  8,  malleus;  9,  long  handle  of  mal- 
leus, attached  to  internal  surface  of  tympanic  membrane— it  is  here  represented  as 
strongly  indrawn;  10,  tensor  tympani  muscle;  11,  tympanic  cavity;  12,  Eustachian 
tube;  13,  superior  semicircular  canal;  14,  posterior  semicircular  canal;  15,  exter- 
nal semicircular  canal;  16,  cochlea;  17,  internal  auditory  meatus;  18,  facial  nerve; 
19.  large  petrosal  nerve;  20,  vestibular  branch  of  auditory  nerve;  21,  cochlear 
branch  of  same. 


such  animals  as  the  horse  or  cow,  but  little  use  as  a  collecting' 
organ  for  the  vibrations  of  the  air.  The  meatus  or  auditory 
canal  may  be  regarded  both  as  a  conductor  of  vibrations  and 
as  protective  to  the  middle  ear,  especially  the  delicate  drum- 
head, since  it  is  provided  with  hairs  externally  in  particular, 
and  with  glands  that  secrete  a  bitter  substance  of  an  unctuous 
nature. 

The  Membrana  Tympani  is  concavo-convex  in  form,  and 
having  attached  to  it  the  chain  of  bones  shortly  to  be  noticed, 
is  well  adapted  to  take  up  the  vibrations  communicated  to  it 
from  the  air;  though  it  also  enters  into  sympathetic  vibration 
when  the  bones  of  the  head  are  the  medium,  as  when  a  tuning- 
fork  is  held  between  the  teeth.  Ordinary  stretched  membranes 
have  a  fundamental  (self-tone,  proper  tone)  tone  of  their  own, 
to  which  they  respond  more  readily  than  to  others. 


HEARING.  559 

If  such  held  for  the  membrana  tympani,  it  is  evident  that 
certain  tones  would  be  heard  better  than  others,  and  that  when 


Fig.  402.— Photographic  representation  of  right  membrana  tympani,  viewed  from 
within  (after  Flint  and  Uiidinger).  1.  divided  head  of  malleus;  2.  neck;  3.  handle, 
with  attachment  of  tendon  of  tensor  tympani;  4,  divided  tendon;  5.  6,  long  handle 
of  malleus;  7,  outer  radiating  and  inner  circular  fibers  of  tympanic  membrane;  8, 
fibrous  ring  encircling  membrana  tympani;  9, 14, 15,  dentated  fibers  of  Gruber;  10, 
11.  posterior  pocket  connecting  with  malleus;  12,  anterior  pocket;  13,  chorda  tym- 
pani nerve. 

the  fundamental  one  was  produced  the  result  might  be  a  sensa- 
tion unpleasant  from  its  intensity.  This  is  partially  obviated 
by  the  damping-  effect  of  the  auditory  ossicles,  which  also  pre- 
vent after-vibrations. 

Some  suppose  that  what  we  denominate  shrill  or  harsh 
sounds  are,  in  part  at  least,  owing-  to  the  auditory  meatus  hav- 
ing a  corresponding  fundamental  note  of  its  own. 

The  Auditory  Ossicles.— Though  these  small  bones  are  con- 
nected by  perfect  joints,  permitting  a  certain  amount  of  play 


560 


COMPARATIVE  PHYSIOLOGY. 


upon  one  another,  experiment  has  shown  that  they  vibrate  in 
response  to  the  movements  of  the  drum-head  en  masse  ;  though 
the  stapes  has  by  no  means  the  range  of  movement  of  the  han- 
dle of  the  malleus ;  in  other  words,  there  is  loss  in  amplitude, 


Fig.  403.— Section  of  auditory  organ  of  horse  (after  Chauveau).  A,  auditory  canal; 
B,  membrana  tympani;  C,  malleus;  D,  incus;  F,  stapes;  O,  mastoid  cells;  //, 
fenestra  ovalis;  I,  vestibule;  ./,  K,  L,  outline  of  semicircular  canals;  M,  cochlea; 
N,  commencement  of  scala  tympani. 

but  gain  in  intensity.  A  glance  at  Fig.  404  will  show  that  the 
end  attained  by  this  arrangement  of  membrane  and  bony  levers, 
which  may  be  virtually  reduced  to  one  (as  it  is  in  the  frog,  etc.), 
is  the  transmission  of  the  vibrations  to  the  membrane  of  the 
fenestra  ovalis,  through  the  stapes  finally,  and  so  to  the  fluids 
within  the  internal  car.  But  it  might  be  supposed  that,  for  the 
avoidance  of  shocks  and  the  better  adaptation  of  the  apparatus 


HEARING. 


561 


to  its  work,  some  regulative  apparatus,  in  the  form  of  a  nerv- 
ous and  muscular  mechanism,  would  have  been  evolved  in  the 


Fig.  404.— Diagrammatic  representation  illustrating  auditory  processes  (after  Beaunis). 
A,  external  ear;  B,  middle  ear:  C,  internal  ear;  1,  auricle;  2.  external  auditory 
meatus;  3,  tympanum;  4,  membrana  tympani;  5,  Eustachian  tube;  6.  mastoid 
cells;  10,  foramen  rotundum;  11,  foramen  ovale;  12.  vestibule;  13,  cochlea;  14, 
scala  tympani;  15,  scala  vestibuli;  16,  semicircular  canals. 

N.  B. — The  ear  is  so  complicated  an  organ  that  it  is  almost  impossible  to  give  a  dia- 
grammatic representation  of  it  at  once  simple  and  complete  in  a  single  figure. 
A  comparison  of  the  whole  series  of  cuts  is  therefore  desirable.  It  is  essential  to 
understand  how  the  end-organ  within  the  scala  media  is  stimulated. 

higher  groups  of  animals.  Such  is  found  in  the  tensor  tym- 
pani, laxator  tympani,  and  stapedius  muscles,  as  well  as  the 
Eustachian  tube. 

Muscles  of  the  Middle  Ear. — The  tensor  tympani  regulates 
the  degree  of  tension  of  the  drum-head,  and  hence  its  ampli- 
tude of  vibration,  having  a  damping  effect,  and  thus  preventing 
the  ill  results  of  very  loud  sounds. 

Ordinarily,  this  is.  doubtless,  a  reflex  act,  in  which  the  fifth 
is  usually  the  afferent  nerve  concerned.  It  is  well-known  that, 
when  we  are  aware  that  an  explosion  is  about  to  take  place,  we 
are  not  as  much  affected  by  it,  which  would  seem  to  argue  a 
voluntary  power  of  accommodation  ;  but  of  this  we  must  speak 
with  caution. 

According  to  some  authorities  the  laxator  tympani  is  not  a 

36 


562 


COMPARATIVE   PHYSIOLOGY. 


muscle,  but  a  supporting  ligament  for  the  malleus.  The  stape- 
dius, however,  has  the  important  function  of  regulating  the 
movements  of  the  stapes,  so  that  it  shall  not  be  too  violently 
driven  against  the  membrane  covering  the  fenestra  ovalis. 

The  two  muscles,  stapedius  and  tensor,  suggest  the  accom- 
modative mechanism  of  the  iris.  The  motor  nerve  of  the  sta- 
pedius is  derived  from  the  facial;  of  the  tensor,  from  the  tri- 
geminus through  the  otic  ganglion. 

The  Eustachian  Tube. — Manifestly,  if  the  middle  ear  were 
closed  permanently,  its  air  would  gradually  be  absorbed.  The 
drum-head  would  be  thrust  in  by  atmospheric  pressure,  and 
become  useless  for  its  vibrating  function.  The  Eustachian 
tube,  by  communicating  with  the  throat,  keeps  the  external  and 
internal  pressure  of  the  middle  ear  balanced.  Whether  this 
canal  is  permanently  open,  or  only  during  swallowing,  is  as  yet 
undetermined. 

■■:-:-^:^%-":'  ■>-.. 


PlG.  405.— Diagram  intended  to  illustrate  the  processes  of  hearing  (after  Landois)  A  G, 
external  auditory  meatus;  7\  tympanic  membrane:  A",  malleus;  a,  incus;  P,  mid- 
dle ear;  o,  fenestra  ovalis;  r,  fenestra  rotunda;  pi,  seala  tympani;  vt,  scala  vesti- 
Imli;  I'.  vestibule;  8,  saccnle;  U,  utricle;  H,  semicircular  canals;  TE,  Eustachian 
tube.  Long  arrow  indicates  line  of  traction  of  tensor  tympani;  short  curved  one 
that  of  Stapedius. 

One  may  satisfy  himself  that  the  middle  ear  and  pharynx 
communicate,  by  closing  the  nostrils  and  then  distending  the 
upper  air-passages  by  a  forced  expiratory  effort,  when  a  sense 
of  distention  within  the  ears  is  experienced,  owing  to  the  rise 
of  atmospheric  pressure  in  the  tympanum. 


HEARING, 


563 


Fig.  406. — Section  through  one  of  the  coils  of  cochlea  (after  Chauveau).  ST.  scala  tym- 
pani;  SV,  scaja  vestibuli;  CC,  cochlear  canal  (scala  media);  Co,  organ  of  Corti: 
11,  membrane  of  Reissner;  b,  membrana  basilaris;  feo,  lamina  spiralis  ossea:  I, 
membrana  tectoria;  1,  2,  rods  of  Corti;  nc,  cochlear  nerve  with  its  ganglion,  gs. 

Pathological. — Inflammation  of  the  tympanum  may  result 
in  adhesions  of  the  small  bones  to  other  parts  or  to  each  other, 


Fid.  407.— I.  Transverse  section  of  a  turn  of  cochlea.  TI.  Ampulla  of  a  semicircular 
canal  and  its  crista  acoustica;  ap,  auditory  cells,  one  of  which  is  a  hair-cell.  111. 
Diagram  of  labyrinth  of  man.    IV.  Of  bird.    V.  Of  fish.    (After  Landois.i 


564 


COMPARATIVE   PHYSIOLOGY. 


Fig.  408.— Diagrammatic  representation  of  ductus  cochlearis  and  organs  of  Corti  (after 
Landois).  N,  nerve  of  cochlea;-  A',  inner,  and  P,  outer,  hair-cells;  n,  nerve-fibrils 
terminating  in  P;  a,  a,  supporting  cells;  d,  cells  of  sulcus  spiralis;  z.  inner  rod 
of  Corti;  y,  outer  rod  of  Corti;  mb,  membrane  of  Corti  (membrana  tectoria);  o, 
membrana  reticularis;  H,  G,  cells  of  area  toward  outer  wall. 


A.N. 


Coch. 


Fig.  410. 


Fio.  409. 


Fig.  409. — Auditory  epithelium  from  macula  acoustica  of  saccule  of  alligator,  much 
magnified  (after  Schafer).  c.  c.  columnar  hair-cells;  f,f,  fiber  cells;  n,  nerve-fiber 
losing  its  medullary  sheath  and  about  to  terminate  in  "columnar  auditory  cells;  h, 
auditory  hair;  h' ,  base  of  auditory  hairs  split  up  into  fibrils. 

I'ii;.  410. — Diagrammatic  representation  of  distribution  of  auditory  nerve  in  membra- 
nous labyrinth  and  cochlea  (after  Huxley). 


HEARING. 


565 


•or  to  occlusion  of  the  Eustachian  tube  from  excess  of  secretion, 
cicatrices,  etc.,  in  consequence  of  which  the  relations  of  atmos- 
pheric pressure  become  altered,  the  membrana  tympani  being 
indrawn,  and  the  whole  series  of  conditions  on  which  the  nor- 
mal transmission  of  vibrations  depends  disturbed,  with  the 
natural  result,  partial  deafness.  The  hardness  of  hearing-  ex- 
perienced during  a  severe  cold  in  the  head  (catarrh,  etc.)  is 
owing  in  great  part  to  the  occlusion  of  the  Eustachian  tube, 
which  may  be  either  partial  or  complete. 

By  filling  one  or  both  of  the  ears  external  to  the  mem- 
brana tympani  with  cotton-wool,  one  may  satisfy  himself  how 
essential  for  hearing  is  the  vibratory  mechanism,  which  is,  of 
course,  under  such  circumstances  inactive  or  nearly  so ;  hence 
the  deafness. 

When  the  middle  ear  is  not  functionally  active,  it  is  still 
possible,  so  long  as  the  auditory  nerve  is  normal,  to  hear  vibra- 
tions of  a  body  (as  a  tuning-fork)  held  against  the  head; 
though,  as  would  be  expected,  discrimination  as  to  pitch  is 
very  imperfect. 

Auditory  impulses  originate  within  the  inner  ear — that  is 
to  say,  in  the  vestibule  and  possibly  the  semicircular  canals, 
but  especially  in  the  cochlea.     It  is  to  he  remembered  that  the 


Fig.  til.— Diagram  intended  to  illustrate  relative  position  of  various  parts  of  ear  (after 
Huxley).  K.  M,  external  auditory  meatus;  Tij.  .1/.  tympanic  membrane;  /'>/.  tvm- 
panum;  Mall,  mallens;  Tnc,  incus;  Stp,  stapes;  F. o,  fenestra  ovalis;  F.r,  fenes- 
tra rotunda;  Si*,  Eustachian  tube;  M.  L,  membranous  labyrinth,  only  one  of  the 
semicircular  canals  and  its  ampulla  being  represented;  Sea.  V,Sca.  T.  Sea.  \f, 
scahc  of  cochlea,  represented  as  straight  umcoiled). 


566  COMPARATIVE   PHYSIOLOGY. 

whole  of  the  end-organ  concerned  in  hearing  is  bathed  by  endo- 
lymph ;  and  that  the  vibrations  of  the  latter  are  originated  by 
corresponding  vibrations  of  the  perilymph,  which  again  is  sent 


Fig.  412.—  Photographic  diagram  of  labyrinth  (after  Flint  and  Rudinger).  Upper  fig- 
ure: 1,  utricle;  2,  saccule;  3,5,  membranous  cochlea;  4,  canalis  reuniens;  6,  semi- 
circular canals.  Lower  figure:  1,  utricle;  2,  saccule;  3,4,6,  ampulla?;  5,7.8,9, 
semicircular  canals;  10,  auditory  nerve  (partly  diagrammatic);  11, 12, 13, 14, 15,  dis- 
tribution of  branches  of  nerve  to  vestibule  and  semicircular  canals. 

into  oscillation  by  the  movements  of  the  stapes  against  the 
membrane  covering  the  fenestra  ovalis ;  so  that  the  vibrations 
thus  set  up  without  the  membranous  labyrinth  are  ti*ans- 
formed  into  similar  ones  within  the  vestibule  and  the  scala 
vestibuli,  and  end,  after  passing  over  the  scala  tympani,  against 
the  membrane  of  the  fenestra  rotunda.  The  cochlear  canal 
may  be  regarded  as  the  seat  of  the  most  important  part  of  the 


HEARING. 


567 


organ  of  hearing,  and  answers  to  the  macula  lutea  of  the  eye 
in  many  respects. 

The  function  of  the  organ  of  Corti  is  unknown. 

The  structure  of  the  ampullae  of  the  semicircular  canals, 
and  other  parts  of  the  lahyrinth  besides  those  specially  con- 


Fig.  413.— Distribution  of  cochlear  nerve  in  spiral  lamina  of  anteroinferior  part  of 
cochlea  of  right  ear  (after  Sappey).  1,  trunk  of  cochlea  nerve;  2,  membranous 
zone  of  spiral  lamina;  3,  terminal  expansion  of  cochlear  nerve  exposed  through- 
out by  removal  of  superior  plate  of  lamina  spiralis;  4,  orifice  of  communication 
between  scala  tympani  and  scala  vestibuli. 

sidered,  with  their  peculiar  hair-cells,  suggests  an  auditory 
function  ;  but  what  that  may  be  is  as  yet  quite  undetermined. 
It  has  been  thought  that  the  parts,  other  than  the  cochlea,  are 
concerned  with  the  appreciation  of  noise,  or  perhaps  the  in- 
tensity of  sounds  ;  but  this  is  a  matter  of  pure  speculation. 


AUDITORY    SENSATIONS,    PERCEPTIONS,    AND 
JUDGMENTS. 

We  have  thus  far  been  concerned  with  the  conduction  of 
the  aerial  vibrations  that  are  the  physical  cause  of  hearing  ; 
but  before  we  can  claim  to  have  "  heard  "  a  word  in  the  highest 
sense,  certain  processes,  some  of  them  physiological  and  some 
psychical,  take  place,  as  in  the  case  of  vision  ;  hence  we  may 
speak  of  the  affection  of  the  end-organ  or  of  auditory  impulses, 
and  of  the  processes  by  which  these  become,  by  the  mediation 
of  the  brain,  auditory  sensations,  and  when  brought  under  the 
cognizance  of  the  mind  as  auditory  perceptions  and  judg- 
ments. 


568 


COMPARATIVE  PHYSIOLOGY. 


Auditory  Judgments. — Such  are  much  more  frequently  erro- 
neous than  are  our  visual  judgments,  whether  the  direction  or 
the  distance  of  the  sound  he  considered.  As  in  the  case  of  the 
eye,  the  muscular  sense,  from  accommodation  of  the  vibratory 
mechanism,  may  assist  our  judgments,  being  aided  by  our 
stored  past  experiences  (memory)  according  to  the  law  of  asso- 
ciation. Sounds  are,  however,  always  referred  to  the  world 
without  us.  The  animals  with  movable  ears  greatly  excel  man 
in  estimating  the  direction,  if  not  the  distance,  of  sounds.  There 
are  few  physiological  experiments  more  amusing  than  those 
performed  on  a  person  blindfolded,  when  attempting  to  deter- 
mine either  the  distance  or  the  direction  of  a  sounding  tuning- 
fork,  so  gross  are  the  errors  made. 

One  who  makes  such  observations  on  others  may  notice  that 
most  persons  move  the  ears  slightly  when  attempting  to  make 
the  necessary  discriminations,  which  of  itself  tends  to  show  how 
valuable  mobility  of  these  organs  must  be  to  those  animals  that 
have  it  highly  developed. 


SPECIAL   CONSIDERATIONS. 

Comparative. — Among  invertebrates  steps  of  progressive  de- 
velopment can  be  traced.     Thus,  in  certain  of  the  jelly-fishes 

we  find  an  auditory  vesicle  (Fig. 
414)  inclosing  fluid  provided  with 
one  or  more  otoliths  or  calcareous 
nodules  and  auditory  cells  with  at- 
tached cilia,  the  whole  making  up 
an  end-organ  connected  with  the 
auditory  nerve.  A  not  very  dis- 
similar arrangement  of  parts  exists 
in  certain  mollusks  (Fig.  415).  The 
vesicle  may  lie  on  a  ganglion  of 
the  central  nervous  system.  On 
the  other  hand,  the  vesicle  may  be 
open  to  the  exterior,  as  in  decapod 
crustaceans  ;  and  the  otoliths  be  re- 
placed by  grains  of  sand  from  with- 
out. It  is  difficult  to  decide  what 
the  function  of  otoliths  may  be  in 


Fie.  414.— Auditory  vesicle  of  Gery- 
onia  (Garmarin a)  seen  from  the 
surface  (after  O.  and  R.  Hert- 
wigV  N  and  N',  the  auditory 
nerves:  Ot,  otolith;  llz,  audito- 
ry cells;  ////,  auditory  cilia  (type  mammals  ;  but  there  seems  to  be 
of   the  auditory  organ   of  the 


TrachymedUBdi) . 


little  reason  to  doubt  that  they  com- 


HEARING. 


569 


municate  vibrations  in  the  invertebrates.  When  the  cephal- 
opod  mollusks,  with  their  highly  developed  nervous  system, 
are  reached,  we  find  a  membranous  and  cartilaginous  labyrinth. 
Among  vertebrates  the  different  parts  of  the  mammalian 
ear  are  found  in  all  stages  of  development.  The  outer  ear  may 
be  wholly  wanting,  as  in  the  frog,  or  it  may  exist  as  a  meatus 
only,  as  in  birds.  The  tympanic  cavity  is  wanting  in  snakes. 
Most  fishes  have  a  utricle  and  three  semicircular  canals,  but  some 


Fig.  415. — Auditory  vesicle  of  a  heteropod  mollusk  (Pterotrachea)  (after  Claus).  V, 
auditory  nerve;  Of,  otolith  in  fluid  of  vesicle;  Wz,  ciliated  cells  on  inner  wall  of 
vesicle;  Hz,  auditory  cells;   Cz,  central  cells. 

have  only  one  ;  and  the  lowest  of  this  group  have  an  ear  not 
greatly  removed  from  the  invertebrate  type,  as  may  be  seen  in 
the  lamprey,  which  has  a  saccule  with  auditory  hairs  and  oto- 
liths, in  communication  with  two  semicircular  canals.  Most 
of  the  amphibia  are  without  a  membrana  tympani.  The  frog 
has  (1)  a  membrana  tympani  communicating  with  the  inner  ear 
by  (2)  a  bony  and  cartilaginous  lever  (columella  auris),  and  (3) 
an  inner  ear  consisting  of  three  semicircular  canals,  a  saccule 
and  utricle  containing  many  otoliths,  and  a  small  dilatation  of 
the  vestibule,  which  may  indicate  an  undeveloped  cochlea. 
The  membranous  labyrinth  is  contained  in  a  periotic  capsule, 
partly  bony  and  partly  cartilaginous,  which  is  supplied  with 


570 


COMPARATIVE  PHYSIOLOGY. 


Fig.  416.— Otoliths  from  various  animals  (after  Eiidinger).  1,  from  goat;  2,  herring; 
3,  devil-fish;  4,  mackerel;  5,  flying-fish;  6,  pike;  7,  carp;  8,  ray;  9,  shark;  10, 
grouse. 


G.C. 


Fio.  417.— Transverse  section  through  head  of  foetal  sheep,  in  region  of  hind-brain,  to 
illustrate  development  of  ear  (after  BOtteher).  //.  B,  hind-brain;  N,  auditory 
nerve;  V.  JB,  vertical  semicircular  canal;  (!(',  canal  of  cochlea;  Ii.  V,  recessus 
vestibuli;  G,  C,  auditory  ganglion;  (/',  terminal  portion  of  auditory  nerve. 


HEARING.  571 

perilymph.  There  is  a  fenestra  ovalis,  but  not  a  fenestra  ro- 
tunda, though  the  latter  is  present  in  reptiles.  In  crocodiles 
and  birds  the  cochlea  is  tubular,  straight,  and  divided  into  a 
scala  tympani  and  a  scala  vestibuli.  The  columella  of  lower 
forms  still  persists.  In  birds  and  mammals  the  bone  back  of 
the  ear  is  hollowed  out  to  some  extent  and  communicates  with 
the  tympanum.  Except  among  the  very  lowest  mammals 
(Echidna),  the  ear  is  such  as  has  been  described  in  detail 
already. 

Evolution. — The  above  brief  description  of  the  auditory  organ 
in  different  groups  of  the  animal  kingdom  will  suffice  to  show 
that  there  has  been  a  progressive  development  or  increasing 
differentiation  of  structure,  while  the  facts  of  physiology  point 
to  a  corresponding  progress  in  function — in  other  words,  there 
has  been  an  evolution.  No  doubt  natural  selection  has  played 
a  great  part.  It  has  been  suggested  that  this  is  illustrated  by 
cats,  that  can  hear  the  high  tones  produced  by  mice,  which 
would  be  inaudible  to  most  mammals  ;  and,  as  the  very  exist- 
ence of  such  animals  must  depend  on  their  detecting  their  prey, 
it  is  possible  to  understand  how  this  principle  has  operated  to 
determine  even  what  cats  shall  survive.  The  author  has  noticed 
that  terrier  dogs  also  have  a  very  acute  sense  of  hearing,  and 
they  also  kill  rats,  etc.  But,  unless  it  be  denied  that  the  im- 
provement from  use  and  the  reverse  can  be  inherited,  this  factor 
must  also  be  taken  into  the  account. 

There  seem  to  be  great  differences  between  hearing  as  it  exists 
in  man  and  in  lower  forms.  Birds,  and  at  least  some  horses, 
possibly  some  cats  and  dogs,  like  music,  and  give  evidence  of 
the  possession  of  a  sense  of  rhythm,  as  evidenced  by  the  conduct 
of  the  steed  of  the  soldier.  On  the  other  hand,  some  dogs  seem  to 
greatly  dislike  music.  Certain  animals  that  appear  to  be  devoid 
of  true  hearing,  as  spiders,  are  nevertheless  sensitive  to  aerial 
vibrations  ;  whether  by  some  special  undiscovered  organ  or 
through  the  general  cutaneous  or  other  kind  of  sensibility  is 
unknown.  It  also  seems  to  be  more  than  probable  that  some 
groups  of  insects  can  hear  sounds  quite  inaudible  to  us,  though 
by  what  organs  is  in  great  measure  unknown. 

The  so-called  musical  ear  differs  from  the  non-musical  in 
the  ability  to  discriminate  differences  in  pitch  rather  thau  in 
quality;  in  fact,  that  one  defective  in  the  former  power  may 
possess  the  latter  in  a  high  degree  is  a  fact  that  has  been  some- 
what lost  sight  of,  both  theoretically  and  practically.     It  does 


572  COMPARATIVE  PHYSIOLOGY. 

not  at  all  follow  that  one  with  little  capacity  for  tune  may  not 
have  the  qualifications  of  ear  requisite  to  make  a  first-rate  elo- 
cutionist. Following  custom,  we  have  spoken  as  though  certain 
defects  and  their  opposites  depended  on  the  ear,  but  in  reality 
we  can  not,  in  the  case  of  man  at  all  events,  affirm  that  such  is 
the  case ;  indeed,  it  seems,  on  the  whole  more  likely  that  they 
are  cerebraF  or  mental.  Auditory  discriminations  seem  to  be 
equally  if  not  more  susceptible  of  improvement  by  culture  than 
visual  ones,  especially  in  the  case  of  the  young. 

A  "  good  ear  "  seems  to  depend  in  no  small  degree  on  mem- 
ory of  sounds,  though  the  latter  may  again  have  its  basis  in 
the  auditory  end-organs  or  in  the  cerebral  cortex,  as  concerned 
in  hearing.  The  necessity  for  the  close  connection  between  the 
co-ordinations  of  the  laryngeal  apparatus  in  singing  and  speak- 
ing and  the  ear  might  be  inferred  from  the  fact  that  many  ex- 
cellent musicians  are  themselves  unable  to  vocalize  the  music 
they  perfectly  appreciate. 

Synopsis  of  the  Physiology  of  Hearing.— The  ear  can  appre- 
ciate differences  in  pitch,  loudness,  and  quality  of  sounds, 
though  whether  different  parts  of  the  inner  ear  are  concerned  in 
these  discriminations  is  unknown.  Hearing  is  the  result  of  a 
series  of  processes,  having  their  physical  counterpart  in  aerial 
vibrations,  which  begin  in  the  end-organ  in  the  labyrinth  and 
terminate  in  the  cerebral  cortex.  We  recognize  conducting 
apparatus  which  is  membranous,  bony,  and  fluid.  The  auditory 
nerve  conveys  the  auditory  impulses  to  the  brain,  though  ex- 
actly what  terminal  cells  are  concerned  and  how  in  originating 
them  must  be  regarded  as  undetermined.  The  essential  part  of 
the  organ  of  hearing  is  bathed  by  endolymph,  and  the  princi- 
pal part  (in  mammals)  is  within  the  cochlear  canal.  Man's 
power  to  locate  sounds  is  very  imperfect.  The  auditory  brain 
center  (or  centers)  has  not  been  definitely  located.  Compara- 
tive anatomy  and  physiology  point  clearly  to  a  progressive 
development  of  the  sense  of  hearing. 


THE   SENSES   OF   SMELL  AND   TASTE. 


SMELL. 


The  nose  internally  may  be  divided  into  a  respiratory  and 
an  olfactory  region.  The  latter,  which  corresponds,  of  course, 
with  the  distribution  of  the  olfactory  nerve,  embraces  the  upper 
and  part  of  the  middle  turbinated  bone  and  the  upper  part  of 
the  septum,  all  of  which  differ  in  microscopic  structure  from 
the  respiratory  region.  Among  the  ordinary  cylindrical  epi- 
thelium of  the  olfactory  region  are  found  peculiar  hair-cells 
highly  suggestive  of  those  of  the  labyrinth  of  the  ear,  and 


Fig.  418.— Parts  concerned  in  smell  (after  Hirschfeld).     1,  olfactory  ganglion  and 
nerves;  2,  branch  of  nasal  nerve,  distributed  over  the  turbinated  bones." 


which  are  to  be  regarded  as  the  en  d-  organs  of  smell.  If  arom  atic 
bodies  be  held  before  the  nose,  and  respiration  suspended,  they 
will  not  be  recognized  as  such,  and  it  is  well  known  that  sniff- 


574 


COMPARATIVE  PHYSIOLOGY. 


ing  greatly  assists  the  sense  of  smell.     Again,  if  fluids,  such,  as 
eau  de  Cologne,  he  held  in  the  nose,  their  aroma  is  not  detected ; 

and  immediately  after  water  has 
heen  kept  in  the  nostrils  for  a  few 
seconds,  it  may  he  noticed  that 
smell  is  greatly  hlunted.  Such  is 
the  case  also  when  the  mucous 
membrane  is  much  swollen  from 
a  cold.  There  can  be  no  doubt 
that  the  presence  of  fluid  in  the 
above  cases  is  injurious  to  the  del- 
icate hair-cells,  and  that  smell  is 
dependent  upon  the  excitation  of 
these  cells  by  extremely  minute 
particles  emanating  from  aromatic 
bodies. 

When  ammonia  is  held  before 
the  nose,  a  powerful  sensation  is 

Fig.  419.— End-organs  concerned  m  '         * 

smell  (after  koiiiker).    i,  from   experienced ;  but  this  is  not  smell 

frog — a,   epithelial  cell  of  the  _,       .  .  -. 

olfactory  area;  b,  olfactory  cell,    proper,   but  an  affection  ot   orcti- 
i"?SSS5S*Sf5?SSSY£SSS   *ary  sensation,  owing  to  stimula- 

of  varicose  fibers.     3,  olfactory    ^  f  ^      terminals  of  the   fifth 

cell  of  sheep. 

nerve.  It  is  possible  that  the  audi- 
tory nerve  may  also  participate,  though  certainly  not  so  as  to 
produce  a  pure  sensation  of  smell. 

Like  the  other  sense-organs,  that  of  smell  is  readily  fa- 
tigued ;  and  perhaps  the  satisfaction  from  smelling  a  bouquet 
of  mixed  flowers  is  comparable  to  viewing  the  same,  one  scent 
after  another  being  perceived,  and  no  one  remaining  predomi- 
nant. 

Our  judgment  of  the  position  of  bodies  possessing  smell  is 
less  perfect  even  than  for  those  emitting  sounds ;  but  we  always 
project  our  sensations  into  the  outer  world,  never  referring  the 
object  to  the  nose  itself.  Subjective  sensations  of  smell  are 
rare  in  the  normal  subject,  though  common  enough  among 
the  diseased,  as  is  complete  or  partial  loss  of  smell.  It  has 
been  found  that  injury  to  the  fifth  nerve  interferes  with  smell, 
which  is  probably  due  to  trophic  changes  in  the  olfactory 
region. 

Comparative.— The  investigation  of  the  senses  in  the  lower 
forms  of  life  is  extremely  difficult,  and  in  the  lowest  presents 
almost  insurmountable  obstacles  to  the   physiologist    because 


SENSES   OF   SMELL   AXD   TASTE.  575 

their  psychic  life  is  so  far  removed  from  our  own  in  terms  of 
which  we  must  interpret,  if  at  all. 

The  earliest  form  of  olfactory  organ  appears  to  be  a  depres- 
sion lined  with  special  cells  in  connection  with  a  nerve,  which, 
indeed,  suggests  the  embryonic  beginnings  of  the  olfactory 
organ  in  vertebrates,  as  an  involution  (pit)  on  the  epithelium 
of  the  head  region.  It  would  appear  that  we  must  believe  that 
in  some  of  the  lower  forms  of  invertebrates  the  senses  of  smell 
and  taste  are  blended,  or  possibly  that  a  perception  results 
which  is  totally  different  from  anything  known  to  us.  The 
close  relation  of  smell  and  taste,  even  in  man,  will  be  referred 
to  presently,  There  are,  perhaps,  greater  individual  differences 
in  sensitiveness  of  the  nasal  organ  among  mankind  than  of  any 
other  of  the  sense-organs.  Women  usually  have  a  much  keener 
perception  of  odors  than  men.  The  sense  of  smell  in  the  dog 
is  well  known  to  be  of  extraordinary  acuteness ;  but  there  are 
not  only  great  differences  among  the  various  breeds  of  dogs, 
but  among  individuals  of  the  same  breeds;  and  this  sense  is 
being  constantly  improved  by  a  process  of  "  artificial  selection  " 
on  the  part  of  man,  owing  to  the  institution  of  field  trials  for 
setters  and  pointers,  the  best  dogs  for  hunting  (largely  deter- 
mined by  the  sense  of  smell)  being  used  to  breed  from,  to  the 
exclusion  of  the  inferior  in  great  part.  Our  own  power  to 
think  in  terms  of  smell  is  very  feeble,  and  in  this  respect  the 
dog  and  kindred  animals  probably  have  a  world  of  their  own 
to  no  small  extent.  Their  memory  of  smells  is  also  immeasur- 
ably better  than  our  own.  A  dog  has  been  known  to  detect  an 
old  hat,  the  property  of  his  master,  that  had  been  given  away 
two  years  before,  as  evidenced  by  his  recovering  it  from  a  re- 
mote place. 

The  importance  of  smell  as  a  guide  in  the  selection  of  food, 
in  detecting  the  presence  of  prey  or  of  enemies,  etc.,  is  very 
obvious.  By  culture  some  persons  have  learned  to  distinguish 
individuals  by  smell  alone,  like  the  dog,  though  to  a  less  degree. 

TASTE. 

The  tongue  is  provided  with  peculiar  modifications  of  epi- 
thelial cells,  etc.,  known  as  papillae  and  taste-buds  which  may 
be  regarded  as  the  end-organs  of  the  glosso-pharyngeaJ  and 
lingual  nerves ;  though  that  these  all,  especially  the  taste-buds, 
are  concerned  with  taste  alone  seems  more  than  doubtful.     In 


576 


COMPARATIVE   PHYSIOLOGY, 


certain  animals  with  rough,  tongues,  the  papillae,  certain  of 
them  at  least,  answer  to  the  hairs  of  a  brush  for  the  cleansing 
and  general  preservation  of  the  coat  of  the  animal  in  good  con- 
dition. We  may,  perhaps,  speak  of  certain  fundamental  taste- 
perceptions,  such  as  siceet,  bitter,  acid,  and  saline.  Certainly 
the  natural  power  of  gustatory  discrimination  is  considerable; 


Fig.  420.— Papillae  of  tongue  (after  Sappey).    1,  circumvallate  papillae;  3,  fungiform 
papillae;  I,  filiform  papillae;  0,  glands  at  base  of  tongue;  7,  tonsils. 

and.  as  in  the  case  of  tea-tasters,  capable  of  extraordinary  culti- 
vation.    All  parts  of  the  tongue  are  not  equally  sensitive,  nor 


SENSES  OF  SMELL  AND   TASTE. 


577 


is  taste-sensation  confined  entirely  to  the  tongue.  It  can  be 
shown  that  the  back  edges  and  tip  of  the  tongue,  the  soft  palate, 
the  anterior  pillars  of  the  fauces,  and  a  limited  portion  of  the 
back  part  of  the  hard  palate,  are  concerned  in  tasting.  Making 
allowances  for  individual  differences,  it  may  be  said  that  the 
back  of  the  tongue  appreciates  best  bitter  substances,  the  tip, 
sweet  ones,  and  the  edges  acids. 

If  any  substance  with  a  decided  taste  be  placed  upon  the 
tongue  when  wiped  quite  dry,  it  can  not  be  tasted  at  all,  show- 
ing that  solution  is  essential. 

If  a  piece  of  apple,  another  of  potato,  and  a  third  of  onion, 
be  placed  upon  the  tongue  of  a  person  blindfolded,  and  with 
the  nostrils  closed,  he  will  not  be  able  to  distinguish  them, 
showing  that  the  senses  of  smell  and  of  taste  are  related ;  or, 
perhaps,  it  may  be  said  that  much  that  we  call  tasting  is  in 
large  part  smelling.  When  the  electrodes  from  a  battery  are 
placed  on  the  tongue,  a  sensation  of  taste  is  aroused,  described 
differently  by  different  persons ;  also  when  the  tongue  is  quick- 
ly tapped,  showing  that,  though  taste  is  usually  the  result  of 
chemical  stimulation,  it  may  be  excited  by  such  as  are  electrical 
or  mechanical. 

But  it  is  not  to  be  forgotten  that  we  have  usually  no  pure 
gustatory  sensations,  but  that  these  are  necessarily  blended 


Fig.  421. 


Fig.  483. 


Fig.  421.— Medium-sized  circumvallate  papilla  (after  Sappcv). 

Fig.  422.— Various  kinds  of  papilhe  cafter  Sappey).    1.  fungiform;  2.  3, 4.  5  6  filiform- 
7,  hemispherical  papillae. 


with  those  of  common  sensation,  temperature,  etc..  and  that  our 
.-judgments  must,  in  the  nature  of  the  case,  be  based  upon  highly 

37 


578 


COMPARATIVE  PHYSIOLOGY. 


complex  data,  even  leaving  out  of  account  other  senses,  such  as 
vision. 

The  glosso-pharyngeal  is  the  principal  nerve  for  the  back  of 
the  tongue,  and  for  the  tip  the  lingual  ;  or  according  to  some 
special  fibers  in  this  nerve,  derived  from  the  chord  tympani. 

It  is  worthy  of  note  that  temperature  has  much  to  do  with 
gustatory  sensations,  a  very  low  or  a  very  high  temperature 


Fig.  423. — Taste-buds  from  tongue  of  rabbit  (after  Engelmann). 

being  fatal  to  nice  discrimination,  and,  as  would  be  expected,  a 
temperature  not  far  removed  from  "  body-heat  "  (40°  C)  is  the 
most  suitable. 

A  certain  amount  of  pressure  is  favorable  to  tasting,  as  any 
one  may  easily  determine  by  simply  allowing  some  solution  of 
quinine  to  rest  on  the  tongue,  and  comparing  the  sensation  with 
that  resulting  when  the  same  is  rubbed  into  the  organ  ;  hence 
the  importance  of  the  movements  of  the  tongue  in  appreciating 
the  sapid  qualities  of  food. 

Comparative. — Among  the  lowest  forms  of  life  it  is  extremely 
difficult  to  determine  to  what  extent  taste  and  smell  exist  sepa- 
rately or  at  all,  as  we  can  conceive  of  them.  The  differentia- 
tion between  ordinary  tactile  sensibility  and  these  senses  has 
no  doubt  been  very  gradually  effected.  Observations  on  our 
domestic  animals  show  that  their  power  of  discrimination  by 
taste  as  well  as  by  smell  is  very  pronounced,  though  their  likes 
and  dislikes  are  so  different  from  our  own  in  many  instances. 
At  the  same  time  we  find  that  they  often  coincide,  and  it  is  not 
unlikely  that  a  dog's  power  of  discriminating  between  a  good 
beefsteak  and  a  poor  one  is  quite  equal  if  not  superior  to  man's, 
and  certainly  so  if  his  sense  of  taste,  as  in  the  human  subject,  is 
developed  in  proportion  to  his  smelling  power. 


THE   CEREBRO-SPINAL   SYSTEM   OF   XEEVES. 

I.  SPINAL   NERVES. 

These  (thirty-one  pairs),  which  leave  the  spinal  cord  through 
the  intervertebral  foramina,  are  mixed  nerves — i.  e.,  their  mam 
trunks  consist  of  motor  and  sensory  fibers.  But  before  they 
enter  the  spinal  cord  they  separate  into  two  groups,  which  are 


Fig.  424. — Diagram  of  roots  of  spinal  nerve  illustrating:  effects  of  section  (after  Dal- 
ton).    The  dark  regions  indicate  the  degenerated  parts. 

known  as  the  anterior  or  motor  and  the  posterior  or  sensory 
roots,  which  make  connection  with  the  anterior  and  posterior 
gray  horns  respectively. 

These  facts  have  been  established  by  a  few  simple  but  im- 
portant physiological  experiments,  which  will  now  be  briefly 
described  :  1.  Stimulation  of  the  peripheral  end  of  a  spinal 
nerve  gives  rise  to  muscular  movements  ;  while  stimulation  of 
its  central  end  causes  pain.  2.  Upon  section  of  the  anterior 
root,  stimulation  of  its  central  end  gives  negative  results  ;  but 
of  its  peripheral  end  causes  muscular  movements.  3.  After 
section  of  the  posterior  root  stimulation  of  the  distal  end  is  fol- 
lowed by  no  sensory  or  motor  effects  ;  of  its  central  end,  by 
sensory  effects  (pain). 

These  experiments  show  clearly  that  the  anterior  roots  are 
motor,  the  posterior  sensory,  and  the  main  trunk  of  the  nerve 
made  up  of  mixed  motor  and  sensory  fibers. 


580  COMPARATIVE  PHYSIOLOGY. 

Exception. — It  has  been  found  that  sometimes  stimulation  of 
the  peripheral  end  of  the  anterior  root  has  given  rise  to  pain, 
an  effect  which  disappears  if  the  posterior  root  be  cut.  From 
this  it  is  inferred  that  certain  sensory  fibers  turn  up  into  the 
anterior  root  a  certain  distance.  Such  are  termed  "  recurrent 
sensory  fibers." 

Additional  Experiments.— 1.  It  is  found  that  if  the  anterior 
root  be  cut,  the  fibers  below  the  point  of  section  degenerate, 
while  those  above  it  do  not.  2.  On  the  other  hand,  when  the 
posterior  root  is  divided  above  the  ganglion,  the  fibers  toward 
the  cord  degenerate,  while  those  on  either  side  of  the  ganglion 
do  not.  From  these  experiments  it  is  inferred  that  the  cells  of 
the  posterior  ganglion  are  essential  to  the  nutrition  of  the  sen- 
sory fibers,  and  those  of  the  anterior  horn  of  the  cord  to  the 
motor  fibers. 

Pathological. — Pathology  teaches  the  same  lesson,  for  it  is 
observed  that,  when  there  is  disease  of  the  anterior  gray  cornua, 
degeneration  of  motor  fibers  is  almost  sure  to  follow.  These 
cells,  whether  in  the  ganglion  or  the  anterior  horn,  have  been 
termed  "trophic."  It  is  true,  the  functions  of  the  ganglia  on 
the  posterior  roots,  other  than  those  just  indicated,  are  un- 
known ;  on  the  other  band,  the  cells  of  the  anterior  horn  are 
distinctly  motor  in  function.  To  assume,  then,  that  the  cells  of 
the  ganglion  are  exclusively  trophic,  with  the  evidence  now 
before  us,  would  be  premature. 

The  view  we  have  presented  of  the  relation  of  the  nervous 
system  makes  all  cells  trophic  in  a  certain  sense  ;  and  we  think 
the  view  that  certain  cells  or  certain  fibers  are  exclusively  tro- 
phic must,  as  yet,  be  regarded  as  an  open  question. 

It  is  important,  however,  to  recognize  that  certain  connec- 
tions between  the  parts  of  the  nervous  system,  and  indeed  all 
of  the  tissues,  are  essential  for  perfect  "nutrition,"  if  we  are  to 
continue  the  use  of  that  term  at  all. 

II.  THE    CRANIAL  NERVES. 

These  nerves  have  been  divided  into  nerves  of  special  sense, 
motor,  and  mixed  nerves. 

The  first  class  has  already  been  considered,  with  the  senses 
to  which  they  belong. 

The  physiology  of  the  cranial  nerves  has  been  worked  out 
by  means  of  sections  and  clinico-pathological  investigations. 


THE  CEREBRO-SPINAL  SYSTEM  OP   NERVES.       581 

Speaking  generally,  a  good  knowledge  of  the  anatomy  of  these 
nerves  is  a  great  step  toward  the  mastery  of  what  is  known  of 


Corpus    (anticum 
quadri-  \ 
geminum(poshcum 


Locus  cocruleus 
Eminentia  teres 


Crus  cercbt'lli 
ad  pontem 


Ala  cinerea 
Acccssorius  nucleus 


Brachinm  conjunctivum  anticum 
Brachium  conjunctivum. 
poslicum 


Corpus  rjeiiiculatum 
mediate 


Pedunculus  cerebri 


ad  corpora  qua-} 

drigemina      I    Crus 
ad  mediillam  I  cerebelli 
oblongatam  ] 

VII 


Obex 
Clavd 


Funiculus  cuneatus 
Funiculus  gracilis 


Fig.  425. — Intended  .0  show  especially  the  origin  of  both  deep  and  superficial  cranial 
nerves  (after  Landois).  Roman  characters  are  used  to  indicate  the  nerves  as  they 
emerge,  and  Arabic  figures  their  nuclei  or  deep  origin. 

their  functions,  and  such  will  he  assumed  in  this  chapter,  so  that 
the  student  may  expect  to  find  the  treatment  of  the  siibject 
somewhat  condensed. 

The  Motor-Oculi  or  Third  Nerve. — With  a  deep  origin  in  the 
gray  matter  of  the  floor  and  roof  of  the  aqueduct  of  Sylvius, 
branches  of  distribution  pass  to  the  following  muscles  :  1.  All 
of  the  muscles  attached  to  the  eyeball,  with  the  exception  of  the 
external  rectus  and  the  superior  oblique.  2.  The  levator  pal- 
pebrae.  3.  The  circular  muscle  of  the  iris.  4.  The  ciliary 
muscle.  Both  the  latter  branches  reach  the  muscles  by  the 
ciliary  nerves,  as  they  pass  from  the  lenticular  (ciliary,  ophthal- 
mic) ganglion.     The  relation  of  the  third  nerve,  as  seen  in  the 


582  COMPARATIVE  PHYSIOLOGY. 

changes  of  the  pupil  with  the  movements  of  the  eyeballs,  has 
already  been  noticed. 

Pathological. — It  follows  that  section  or  lesion  of  the  third 
nerve  must  give  rise  to  the  following  symptoms  :  1.  Drooping 
of  the  upper  lid  (ptosis).  2.  Fixed  position  of  the  eye  in  the 
outer  angle  of  the  orbit  (luscitas).  3.  Immobility,  with  the  dila- 
tation of  the  pupil  (mydriasis).     4.  Loss  of  accommodation. 

The  Trochlear  or  Fourth  Nerve. — This  nerve,  arising  in  the 
aqueduct  of  Sylvius,  passes  to  the  superior  oblique  muscle. 

Pathological. — Lesion  of  this  nerve  leads  to  peculiar  changes. 
As  there  is  double  vision,  some  alteration  must  have  occurred 
in  the  usual  position  of  the  globe  of  the  eye,  though  this  is  not 
easily  seen  on  looking  at  a  subject  thus  affected.  The  double 
image  appears  when  the  eyes  are  directed  downward,  and  ap- 
pears oblique  and  lower  than  that  seen  by  the  unaffected  eye. 

The  Abductor  or  Sixth  Nerve. — Arising  on  the  floor  of  the 
fourth  ventricle,  it  passes  to  the  external  rectus  of  the  eyeball, 
thus  with  the  third  and  fourth  nerve  completing  the  innerva- 
tion of  the  external  ocular  muscles  (extrinsic  muscles). 

Pathological. — Lesion  of  this  nerve  causes  paralysis  of  the 
above-mentioned  muscle,  and  consequently  internal  squint 
(strabismus). 

The  Facial,  Portia  Dura,  or  Seventh  Nerve.— It  arises  in  a 
gray  nucleus  in  the  floor  of  the  fourth  ventricle,  and  has  an 
extensive  distribution  to  the  muscles  of  the  face,  and  may  be 
regarded,  in  fact,  as  the  nerve  of  the  facial  muscles,  since  it  sup- 
plies (1)  the  muscles  of  expression,  as  those  of  the  forehead, 
eyelids,  nose,  cheek,  mouth,  chin,  outer  ear,  etc.,  and  (2)  certain 
muscles  of  mastication,  as  the  buccinator,  posterior  belly  of  the 
digastric,  the  stylohyoid,  and  also  (3)  to  the  stapedius,  with 
branches  to  the  soft  palate  and  uvula. 

Pathological. — It  follows  that  paralysis  of  this  nerve  must 
give  rise  to  marked  facial  distortion,  loss  of  expression,  and 
flattening  of  the  features,  as  well  as  possibly  some  deficiency  in 
hearing,  smelling,  and  swallowing.  Mastication  is  difficult, 
and  the  food  not  readily  retained  in  the  mouth.  Speech  is 
affected  from  paralysis  of  the  lips,  etc. 

Secretory  fibers  proceed  (1)  to  the  parotoid  gland  by  the 
superficial  petrosal  nerve,  thence  (2)  to  the  otic  ganglion,  from 
which  the  fibers  pass  by  the  auriculotemporal  nerve  to  the 
gland. 

Gustatory  Fibers. — According  to  some,  the  chorda  tympani 


THE  CEREBROSPINAL  SYSTEM   OF   NERVES.      583 

really  supplies  the  fibers  to  the  lingual  nerve  that  are  concerned 
with  taste. 

It  will  thus  be  seen  that  the  facial  nerve  has  a  great  variety 
of  important  functions,  and  that  paralysis  may  be  more  or  less 
serious,  according  to  the  number  of  fibers  involved. 

The  Trigeminus,  Trifacial,  or  Fifth  Nerve. — This  nerve  has 
very  extensive  functions.  It  is  the  sensory  nerve  of  the  face  : 
but,  as  will  be  seen,  it  is  peculiar,  being  a  combination  of  the 
motor  and  sensory  ;  or,  in  other  words,  has  paths  for  both 
afferent  and  efferent  impulses.  The  motor  and  less  extensive 
division  arises  from  a  nucleus  in  the  floor  of  the  fourth  ventricle. 
The  sensory,  much  the  larger,  seems  to  have  a  very  wide  origin. 
The  nerve-fibers  may  be  traced  from  the  pons  Varolii  through 
the  medulla  oblongata  to  the  lower  boundary  of  the  olivary  body 
and  to  the  posterior  horn  of  the  spinal  cord.  This  origin  sug- 
gests a  resemblance  to  a  spinal  nerve,  the  motor  root  corre- 
sponding to  the  anterior,  and  the  sensory  to  a  posterior  root, 
the  more  so  as  there  is  a  large  ganglion  connected  with  the 
sensory  part  of  the  nerve  within  the  brain-case. 

Efferent  Fibers.— 1.  Motor. — To  certain  muscles  (1)  of  mas- 
tication— temporal,  masseter,  pterygoid,  mylohyoid,  and  the 
anterior  part  of  the  digastric.  2.  Secretory. — To  the  lachrymal 
gland  of  the  ophthalmic  division  of  this  nerve.  3.  Vaso-motor. 
— Probably  to  the  ocular  vessels,  those  of  the  mucous  mem- 
brane of  the  cheek  and  gums,  etc,  4.  Trophic. — From  the  re- 
sults ensuing  on  section  of  this  nerve,  it  has  been  maintained 
that  special  trophic  fibers  pass  in  it.  We  have  discussed  this 
subject  in  an  earlier  chapter. 

Afferent  Fibers. — 1.  Sensory. — To  the  entire  face.  To  par- 
ticularize regions :  1.  The  whole  of  the  skin  of  the  face  and 
that  of  the  anterior  surface  of  the  external  ear.  2.  The  external 
auditory  meatus.  3.  The  mucous  lining  of  the  cheeks,  the  floor 
of  the  mouth,  and  the  anterior  region  of  the  tongue.  4.  The 
teeth  and  periosteum  of  the  jaws.  5.  The  lining  membrane  of 
the  entire  nasal  cavity.  6.  The  conjunctiva,  globe  of  the  eye, 
and  orbit.     7.  The  dura  mater  throughout. 

Many  of  these  afferent  fibers  are,  of  course,  intimately  con- 
cerned with  reflexes,  as  sneezing,  winking,  etc.  Certain  secre- 
tory acts  are  often  excited  through  this  nerve,  as  lachrymation, 
when  the  nasal  mucous  membrane  is  stimulated  :  indeed,  the 
paths  for  afferent  impulses  giving  rise  to  reflexes,  including 
secretion,  are  very  numerous. 


584 


COMPARATIVE  PHYSIOLOGY. 


Gustatory  impulses  from  the  anterior  end  and  lateral  edges 
of  the  tongue  are  conveyed  hy  the  lingual  (gustatory)  branch 
of  this  nerve.     Many  are  of  opinion,  however,  that  the  fibers 
of  the  chorda  tympani,  which  afterward  leave  the  lingual  to 
unite  with  the  facial  nerve,  alone  con- 
vey  such  impressions.      The  subject 
can  not  be  regarded  as  quite  settled. 
Tactile  sensibility  in  the  tongue  is  very 
pronounced,  as  we  have  all   experi- 
enced when  a  tooth,  etc.,  has  for  some 
reason  presented  an  unusual  surface 
quality,  and  become  a  source  of  con- 
stant offense  to  the  tongue. 

The  ganglia  of  the  fifth  nerve,  so 
far  as  the  functions  of  their  cells  are 
concerned,  are  enigmatical  at  present. 
They  are  doubtless  in  some  sense  tro- 
phic at  least.  With  each  of  these  are 
nerve  connections  ("  roots  "  of  the  gan- 
glia), which  seem  to  contain  different 
kinds  of  fibers.  These  ganglia  are 
connected  with  the  main  nerve-centers 
by  both  afferent  and  efferent  nerves, 
and  also  with  the  sympathetic  nerves 
themselves.  Some  regard  the  ganglia 
as  the  representatives  of  the  sympa- 
thetic system  within  the  cranium. 

I.  The  Ciliary  (Ophthalmic,  Len- 
ticular) Ganglion.  —  Its  three  roots 
are  :  1.  From  the  branch  of  the  third 
nerve  to  the  inferior  oblique  muscle 
(motor  root).  2.  From  the  nasal 
branch  of  the  ophthalmic  division  of 
the  fifth.  3.  From  the  carotid  plexus 
of  the  sympathetic.  The  efferent 
branches  pass  to  the  iris,  are  derived 
chiefly  from  the  sympathetic,  and 
cause  dilatation  of  the  pupil.  There 
are  also  vaso-motor  fibers  to  the  choroid,  iris,  and  retina.  The 
afferent  fibers  are  sensory,  passing  from  the  conjunctiva,  cor- 
nea, etc. 

II.   The  Nasal  or  Spheno-Palatine  Ganglion.— The  motor 


Fk;.  426.— Unipolar  cell  from 
Gaeserian  ganglion  (after 
Schwalbe).  N,  N,  N,  nuclei 
of  sheath;  7',  liber  branch- 
ing at  a  node  of  Ranvier. 


THE   CEREBRO-SPINAL  SYSTEM   OF   NERVES.       585 

root  is  dei-ived  from  the  facial  through  the  great  superficial 
petrosal  nerve;  its  sympathetic  root  from  the  carotid  plexus. 
Both  together  constitute  the  vidian  nerve.  It  would  seem  that 
afferent  impulses  from  the  nasal  chambers  pass  through  this 
ganglion.  The  efferent  paths  are  :  1.  Motor  to  the  levator  pa- 
lati  and  azygos  uvulae.  2.  Vaso-motor,  derived  from  the  sym- 
pathetic.    3.  Secretory  to  the  glands  of  the  cheek,  etc. 

III.  The  Otic  Ganglion.— Its  roots  are  :  1.  Motor,  from  the 
third  division.  2.  Sensory,  from  the  inferior  division  of  the 
fifth.  3.  Sympathetic,  from  the  plexus  around  the  meningeal 
artery.  It  makes  communication  with  the  chorda  tympani  and 
seventh,  and  supplies  the  parotid  gland  with  some  fine  fila- 
ments. Motor  fibers  mixed  with  sensory  ones  pass  to  the  tensor 
tympani  and  tensor  palati. 

IV.  The  Submaxillary  Ganglion. — Its  roots  are:  1.  Branch- 
es of  the  chorda  tympani,  from  which  pass  (a)  secretory  fibers  to 
the  submaxillary  and  sublingual  glands,  (b)  vaso-motor  (dilator) 
fibers  to  the  vessels  of  the  same  glands.  2.  The  sympathetic, 
derived  from  the  supei'ior  cervical  ganglion,  passing  to  the  sub- 
maxillary gland.  It  is  also  thought  to  be  the  path  of  vaso-con- 
strictor  fibers  to  the  gland.  3.  The  sensory,  from  the  lingual 
nerve,  supplying  the  gland  substance,  its  ducts,  etc. 

Pathological. — 1.  The  motor  division  of  the  nerve,  when 
the  medium  of  efferent  impulses,  owing  to  central  disorder,  may 
cause  trismus  (locked-jaw)  from  tonic  tetanic  action  of  the  mus- 
cles of  mastication  supplied  by  this  nerve.  2.  Paralysis  of  the 
same  muscles  may  ensue  from  degeneration  of  the  motor  nuclei 
or  pressure  on  the  nerve  in  its  course.  3.  Neuralgia  of  any  of 
the  sensory  branches  may  occur  from  a  great  variety  of  causes, 
and  often  maps  out  very  exactly  the  course  and  distribution  of 
the  branches  of  the  nerve.  4.  Vaso-motor  distui'bances  are  not 
infrequently  associated  with  neuralgia.  Blushing  is  an  evi- 
dence of  the  normal  action  of  the  vaso  motor  fibers  of  the  fifth 
nerve.  5.  A  variety  of  trophic  (metabolic)  disturbances  may 
arise  from  disorder  of  this  nerve,  its  nuclei  of  origin  or  its  gan- 
glia, such  as  grayness  and  loss  of  hair  (imperfect  nutrition), 
eruptions  of  the  skin  along  the  course  of  the  nerves,  etc.  Atro- 
phy of  the  face,  on  one  or  both  sides,  gradual  and  progressive, 
may  occur.  Such  affections  as  well  as  others,  point  in  the  most 
forcible  manner  to  the  influence  of  the  nervous  system  over  the 
metabolism  of  the  body. 

The  Glosso-pharyngeal  or  Ninth  Nerve. — This  nerve,  to- 


586  COMPARATIVE  PHYSIOLOGY. 

gether  with,  the  vagus  and  spinal  accessory,  constitutes  the 
eighth  pair,  or  rather  trio.  Functionally,  however,  they  are 
quite  distinct. 

The  glosso-pharyngeal  arises  in  the  floor  of  the  fourth  ven- 
tricle ahove  the  nucleus  for  the  vagus.  It  is  a  mixed  nerve 
with  efferent  and  afferent  fibers  :  Efferent  fibers,  furnishing 
motor  fibers  to  the  middle  constrictor  of  the  pharynx,  stylo- 
pharyngeus,  levator  palati,  and  azygos  uvulae.  2.  Afferent 
fibers,  which  are  the  paths  of  sensory  impulses  from  the  base 
of  the  tongue,  the  soft  palate,  the  tonsils,  the  Eustachian  tube, 
tympanum,  and  anterior  portion  of  the  epiglottis.  Stimulation 
of  the  regions  just  mentioued  gives  rise  reflexly  to  the  move- 
ments of  swallowing  and  to  reflex  secretion  of  saliva. 

This  nerve  is  also  the  special  nerve  of  taste  to  the  back  of 
the  tongue. 

The  Pneumogastric,  Vagus,  or  Tenth  Nerve.— Most  of  the 
functions  of  this  nerve  have  already  been  considered  in  previous 
chapters. 

In  some  of  the  lower  vertebrates  (sharks)  the  nerve  arises 
by  a  series  of  distinct  roots,  some  of  which  remain  separate 
throughout.  This  fact  explains  its  peculiarities,  anatomical 
and  functional,  in  the  higher  vertebrates.  In  these  there  have 
been  concentration  and  blending,  so  that  what  seems  to  be  one 
nerve  is  really  made  up  of  several  distinct  bundles  of  fibers, 
many  of  which  leave  the  main  trunk  later. 

It  may  be  regarded  as  the  most  complicated  nerve-trunk  in 
the  body,  and  the  distribution  of  its  fibers  is  of  the  most  exten- 
sive character.  Following  our  classification  of  efferent  and 
afferent,  we  recognize : 

1.  Efferent  fibers,  which  are  motor  to  an  extensive  tract  in 
the  respiratory  and  alimentary  regions. 

Thus  the  constrictors  of  the  pharynx,  certain  muscles  of  the 
palate,  the  oesophagus,  the  stomach,  and  the  intestine,  receive 
an  abundant  supply  from  this  source.  By  the  laryngeal  nerves, 
probably  derived  originally  from  the  spinal  accessory,  the  mus- 
cles of  the  larynx  are  innervated.  The  muscles  of  the  trachea, 
bronchi,  etc.,  are  also  supplied  by  the  pneumogastric.  It  is 
probable  that  vaso-motor  fibers  derived  from  the  sympathetic 
run  in  branches  of  the  vagus.  The  relations  of  this  nerve  to 
the  heart  and  lungs  have  already  been  explained. 

2.  Afferent  Fibers. — It  may  be  said  that  afferent  impulses 
from  all  the  regions  to  which  efferent  fibers  are  supplied  pass 


THE  CEREBRO-SPINAL  SYSTEM  OF  NERVES.       587 

inward  by  the  vagus.  One  of  the  widest  tracts  in  the  body 
for  afferent  impulses  giving-  rise  to  reflexes  is  connected  with 
the  nerve-centers  by  the  branches  of  this  nerve,  as  evidenced  by 
the  niany  well-known  phenomena  of  this  character  referable  to 
the  pharynx,  larynx,  lungs,  stomach,  etc.,  as  vomiting,  sneez- 
ing, coughing,  etc.  This  nerve  plays  some  important  part  in 
secretion,  no  doubt,  but  what  that  is  has  not  been  as  yet  well 
established. 

Pathological. — Section  of  both  vagi,  as  might  be  expected, 
leads  to  death,  which  may  take  place  from  a  combination  of 
pathological  changes,  the  factors  in  which  vary  a  good  deal 
with  the  class  of  animals  the  subject  of  experiment.  Thus,  the 
heart  in  some  animals  (dog)  beats  with  great  rapidity  and  tends 
to  exhaust  itself.  In  birds  especially  is  fatty  degeneration  of 
heart,  stomach,  intestines,  etc.,  liable  to  follow. 

Paralysis  of  the  muscles  of  the  larynx  renders  breathing 
laborious.  From  loss  of  sensibility  food  accumulates  in  the 
pharynx  and  finds  its  way  into  the  larynx,  favoring,  if  riot 
actually  exciting,  inflammation  of  the  air-passages. 

But  it  is  not  to  be  forgotten  that  upon  the  views  we  advocate 
as  to  the  constant  influence  of  the  nervous  system  over  all  parts 
of  the  bodily  metabolism,  it  is  plain  that  after  section  of  the 
trunk  of  a  nerve  with  fibers  of  such  wide  distribution  and  va- 
ried functions  the  most  profound  changes  in  so-called  nutrition 
must  be  expected,  as  well  as  the  more  obvious  functional  de- 
rangements ;  or,  to  put  it  otherwise,  the  results  that  follow  are 
in  themselves  evidence  of  the  strongest  kind  for  the  doctrine  of 
a  constant  neuro-metabolic  influence  which  we  advocate.  It 
will  not  be  forgotten  that  the  depressor  nerve,  which  exerts  re- 
flexly  so  important  an  influence  over  blood-pressure,  is  itself 
derived  from  the  vagus. 

The  Spinal  Accessory  or  Eleventh  Nerve!— This  nerve  arises 
from  the  medulla  oblongata  somewhat  far  back,  and  from  the 
spinal  cord  in  the  region  of  the  fifth  to  the  seventh  vertebra. 
Leaving  the  lateral  columns,  its  fibers  run  upward  between  the 
denticulate  ligament  and  the  posterior  roots  of  the  spinal  nerve 
to  enter  the  cranial  cavity,  which  as  they  issue  from  the  cra- 
nium subdivide  into  two  bundles,  one  of  which  unites  with  the 
vagus,  while  the  other  pursues  an  independent  course  to  reach 
the  sterno-mastoid  and  trapezius  muscles,  to  which  they  furnish 
the  motor  supply;  so  that  it  may  be  considered  functionally 
equivalent  to  the  anterior  root  of  a  spinal  nerve.     The  portion 


588  COMPARATIVE  PHYSIOLOGY. 

joining  the  vagus  seems  to  supply  a  large  part  of  the  motor 
fibers  of  that  nerve. 

Pathological. — Tonic  contraction  of  the  flexors  of  the  head 
causes  wry -neck,  and  when  they  are  paralyzed  the  head  is  drawn 
to  the  sound  side. 

The  Hypoglossal  or  Twelfth  Nerve.— It  arises  from  the  low- 
est part  of  the  calamus  scriptorius  and  perhaps  from  the  olivary 
body.  The  manner  of  its  emergence  between  the  anterior  pyra- 
mid and  the  olivary  body,  on  a  line  with  the  anterior  spinal 
roots,  suggests  that  it  corresponds  to  the  latter ;  the  more  so  as 
it,  is  motor  in  function,  though  also  containing  some  vaso-motor 
fibers,  in  all  probability  destined  for  the  tongue.  Such  sensory 
fibers  as  it  may  contain  are  derived  from  other  sources  (vagus, 
trigeminus).  It  supplies  motor  fibers  to  the  tongue  and  the 
muscles,  attached  to  the  hyoid  bone. 

Pathological. — Unilateral  section  of  the  nerve  gives  rise  to 
a  corresponding  lingual  paralysis,  so  that  when  the  tongue  is 
protruded  it  points  to  the  injured  side ;  when  being  drawn  in, 
the  reverse.  Speech,  singing,  deglutition,  and  taste  may  also 
be  abnormal,  owing  to  the  subject  being  unable  to  make  the 
usual  co-ordinated  movements  of  the  tongue  essential  for  these 
acts. 

RELATIONS   OF   THE    CEREBRO-SPINAL   AND    SYMPA- 
THETIC   SYSTEMS. 

No  division  of  the  nervous  system  has  been  so  unsatisfac- 
tory, because  so  out  of  relation  with  other  parts,  as  the  sympa- 
thetic. It  was  also  desirable  to  attempt  to  co-ordinate  the  cere- 
bral and  spinal  nerves  in  a  better  fashion ;  and  various  attempts 
in  that  direction  have  been  made.  Very  recently  a  plan,  by 
which  the  whole  of  the  nerves  issuing  from  the  brain  and  cord 
may  be  brought  into  a  unity  of  conception,  has  been  proposed; 
and,  though  it  would  be  premature  to  pronounce  definitely  as 
yet  upon  the  scheme,  yet  it  does  seem  to  be  worth  while  to  lay 
it  before  the  student,  as  at  all  events  better  than  the  isolation 
implied  in  the  three  divisions  of  the  nerves  which  has  been 
taught  hitherto. 

Instead  of  the  classification  of  nerves  into  efferent  and  affer- 
ent, connected  with  the  anterior  and  the  posterior  horns  of  the 
gray  matter  of  the  spinal  cord,  another  division  has  been  pro- 
posed, viz.,  a  division  of  nerve-fibers  and  their  centers  of  origin 


THE  CEREBRO-SPINAL   SYSTEM   OF   NERVES.       589 

in  the  gray  matter  for  the  supply  of  the  internal  and  the  exter- 
nal parts  of  the  hody — i.  e.,  into  splanchnic  and  somatic  nerves. 
The  centers  of  origin  of  the  splanchnic  nerves  are  referred  to 
groups  of  cells  in  the  gray  matter  of  the  cord  around  the  cen- 


Fig.  427. 


Fig.  428. 


Fig.  427. — Ganglion  cell  from  sympathetic  ganglion  of  frog;  greatly  magnified,  and 
showing  both  straight  and  coiled  fibers  (after  Quain). 

Fig.  428. — Multipolar  ganglion  cells  from  sympathetic  system  of  man,  highly  magni- 
fied (after  Max  Scnuftze).  a,  cell  freed  from  capsule;  b,  inclosed  within  a  "nu- 
cleated capsule.    In  both  the  processes  have  been  broken  away. 


tral  canal ;  while  the  somatic  nerves  spring  from  the  gray  cor- 
nua  and  supply  the  integument  and  the  ordinary  muscles  of 
locomotion,  etc.  The  splanchnic  nerves  supply  certain  muscles 
of  respiration  and  deglutition,  derived  from  the  embryonic 
lateral  plates  of  the  mesoblast;  the  somatic  nerves,  muscles 
formed  from  the  muscle-plates  of  the  same  region. 

It  is  assumed  that  the  segmentation  of  the  vertebrate  and 
invertebrate  animal  is  related;  and  that  segmentation  is  pre- 


590  COMPARATIVE  PHYSIOLOGY. 

served  in  the  cranial  region  of  the  vertebrate,  as  shown  by  the 
nerves  themselves. 

The  afferent  fibers  of  both  splanchnic  and  somatic  nerves 
pass  into  the  spinal  ganglion,  situated  in  the  nerve-root,  which 
may  be  regarded  as  stationary. 

It  is  different  with  the  anterior  roots.  Some  of  the  fibers 
are  not  connected  with  ganglia  at  all ;'  others  with  ganglia  not 
fixed  in  position,  but  occurring  at  variable  distances  from  the 
central  nervous  system  (these  being  the  so-called  sympathetic 
ganglia) :  thus,  the  anterior  root-fibers  are  divisible  into  two 
groups,  both  of  which  are  efferent,  viz.,  ganglionated  and  non- 
ganglionated.  The  ganglionated  belong  to  the  splanchnic  sys- 
tem, and  have  relatively  small  fibers;  the  non-ganglionated 
include  both  somatic  and  splanchnic  nerves,  composing  the 
ordinary  nerve-fibers  of  the  voluntary  striped  muscles  of  res- 
piration, deglutition,  and  locomotion. 

It  would  appear  that  these  now  isolated  ganglia  have  been 
themselves  derived  from  a  primitive  ganglion  mass  situated  on 
the  spinal  nerves;  so  that  the  distinction  usually  made  of  gan- 
glionated and  non-ganglionated  roots  is  not  fundamental. 

A  spinal  nerve  is,  then,  formed  of — 1.  A  posterior  root,  the 
ganglion  of  which  is  stationary  in  position,  and  connected  with 
splanchnic  and  somatic  nerves,  both  of  which  are  afferent.  2. 
An  anterior  root,  the  ganglion  of  which  is  vagrant,  and  con- 
nected with  the  efferent  small-fibered  splanchnic  nerves. 

Among  the  lower  vertebrates  both  anterior  and  posterior 
roots  pass  into  the  same  stationary  ganglion.  Such  is  also  the 
case  in  the  first  two  cervical  nerves  of  the  dog. 

Does  the  above-mentioned  plan  of  distribution,  etc.,  hold  for 
the  cranial  nerves  ? 

Leaving  out  the  nerves  of  special  sense  (olfactory,  optic,  and 
auditory),  the  other  cranial  nerves  maybe  thus  divided:  1.  A 
foremost  group  of  nerves,  wholly  efferent  in  man,  viz.,  the 
third,  fourth,  motor  division  of  the  fifth,  the  sixth,  and  seventh. 
2.  A  hindmost  group  of  nerves  of  mixed  character,  viz.,  the 
ninth,  tenth,  eleventh,  and  twelfth. 

The  nerves  of  the  first  group,  since  they  have  both  large- 
fibered,  non-ganglionated  motor  nerves,  and  also  small-fibered 
splanchnic  efferent  nerves,  with  vagrant  ganglia  (ganglion 
oculomotorii,  ganglion  geniculatum,  etc.),  resemble  a  spinal 
nerve  in  respect  to  their  anterior  roots.  They  also  resemble 
spinal  nerves  as  to  their  posterior  roots,  for  at  their  exit  from 


THE  CEREBRO-SPINAL  SYSTEM   OP  NERVES.       591 

the  brain  tliey  pass  a  gang-lion  corresponding-  to  the  stationary- 
posterior  ganglion  of  the  posterior  root  of  a  spinal  nerve. 
These  being,  however,  neither  in  roots  nor  ganglion  functional, 
are  to  be  regarded  as  the  pbylogenetically  (ancestrally)  degen- 
erated remnants  of  what  were  once  functional  ganglia  and 
nerve-fibers ;  in  other  words,  the  afferent  roots  of  these  nerves 
and  their  ganglia  have  degenerated. 

The  hindmost  group  of  cranial  nerves  also  answers  to  the 
spinal  nerves.  They  arise  from  nuclei  of  origin  in  the  medulla 
and  in  the  cervical  region  of  the  spinal  cord,  directly  continu- 
ous with  corresponding  groups  of  nerve-cells  in  other  parts  of 
the  spinal  cord ;  but  in  these  nerves  there  is  a  scattering  of  the 
components  of  the  corresponding  spinal  nerves.  Cei'tain  pecul- 
iarities of  these  cranial  nerves  seem  to  become  clearer  if  it  be 
assumed  that,  in  the  development  of  the  vertebrate,  degenera- 
tion of  some  region  once  functional  has  occurred,  in  conse- 
quence of  which  certain  portions  of  nerves,  etc.,  have  disap- 
peared or  become  functionless. 

It  is  also  to  be  remembered  thafsa  double  segmentation  ex- 
ists in  the  body,  viz.,  a  somatic,  represented  by  vertebras  and 
their  related  muscles,  and  a  splanchnic  represented  by  visceral 
and  branchial  clefts,  and  that  these  two  have  not  followed  the 
same  lines  of  development ;  so  that  in  comparing  spinal  nerves 
arranged  in  regard  to  somatic  segments  with  cranial  nerves, 
the  relations  of  the  latter  to  the  somatic  muscles  of  the  head 
must  be  considered;  in  other  words,  like  must  be  compared 
with  like. 


THE  VOICE, 


It  is  convenient  to  speak,  in  the  case  of  man,  of  the  singing 
voice  and  the  speaking  voice,  though  there  is  no  fundamental 
difference  in  their  production.  The  voice  of  the  lower  animals 
approximates  the  former  leather  than  the  latter. 

It  is  to  be  remembered  that  sound  is  an  affection  of  the 
nervous  centers  through  the  ear,  as  the  result  of  aerial  vibra- 
tions. 

We  are  now  to  explain  how  such  vibrations  are  caused  by 
the  vocal  mechanisms  of  animals  and  especially  of  man. 

The  toues  of  a  piano  or  violin  are  demonstrably  due  to  the 
vibrations  of  the  strings ;  of  a  clarionet  to  the  vibration  of  its 
reed.  But,  however  musical  tones  may  be  produced,  we  distin- 
guish in  them  differences  in  pitch,  quantity,  and  quality. 

The  pitch  is  dependent  solely  upon  the  number  of  vibrations 
within  a  given  time,  as  one  second;  the  quantity  or  loudness 
upon  the  amplitude  of  the  vibrations,  and  the  quality  upon  the 
form  of  the  vibrations.  The  first  two  scarcely  require  any  fur- 
ther notice ;  but  it  is  rather  important  for  our  purpose  to  under- 
stand clearly  the  nature  of  quality  or  timbre,  which  is  a  more 
complex  matter. 

If  a  note  be  sounded  near  an  open  piano,  it  may  be  observed 
that  not  only  the  string  capable  of  giving  out  the  correspond- 
ing note  passes  into  feeble  vibration,  but  that  several  others 
also  respond.  These  latter  produce  the  overtones  or  partials 
which  enter  into  notes  and  determine  the  quality  by  which  one 
instrument  or  one  voice  differs  from  another.  In  other  words, 
every  tone  is  in  reality  compound,  being  composed  of  a  funda- 
mental tone  and  overtones.  These  vary  in  number  and  in  rela- 
tive strength  with  each  form  of  instrument  and  each  voice; 
and  it  is  now  customary  to  explain  the  differences  in  quality  of 
voices  solely  in  this  way ;  and  this  is,  no  doubt,  correct  in  the 
main. 


THE   VOICE, 


593 


What  are  the  mechanisms  by  which  voice  is  produced  in 
man  ?     Observation  proves  that  the  following  are  essential :  1. 


Fig.  429. 


Fig.  430. 


Fig.  429. — Longitudinal  section  of  human  larynx  (after  Sappey).  1,  ventricle  of  lar- 
ynx; 2,  superior  vocal  cord;  3,  inferior  vocal  cord;  4.  arytenoid  cartilage;  5,  sec- 
tion of  arytenoid  muscle;  6.  6.  inferior  portion  of  cavity  of  larynx:  ',.  section  Of 
posterior  part  of  cricoid  cartilage:  8.  section  of  anterior  part  of  same;  9,  superior 
border  of  cricoid  cartilage;  10,  section  of  thyroid  cartilage;  11.11.  superior  portion 
of  cavity  of  larynx:  12, 13,  arytenoid  gland;  14, 16,  epiglottis;  15, 17,  adipose  tissue: 
18.  section  of  liyoid  bone;  19, 19.  SO,  trachea. 

Pig.  430. — Posterior  aspect  of  muscles  of  human  larynx  (after  Sappey).  1,  posterior 
crico-arytenoid  muscle;  2,  3,  4,  different  fasciculi' of  arytenoid  muscle;  5,  aryteno- 
epiglottidean  muscle. 

A  certain  amount  of  tension  of  the  vocal  cords  (bands).  2.  A 
certain  degree  of  approximation  of  their  edges.  3.  An  expira- 
tory blast  of  air. 

It  will  be  noted  that  these  are  all  conditions  favorable  to  the 
vibration  of  the  vocal  bauds.  The  greater  the  tension  the 
higher  the  pitch ;  and  the  more  occluded  the  glottic  orifice  the 
more  effective  the  expiratory  blast  of  air. 

The  principle  on  which  the  vocal  bands  act  may  be  illus- 
38 


594 


COMPARATIVE   PHYSIOLOGY. 


tratecl  in  the  simplest  way  by  a  well-known  toy,  consisting  of 
an  elastic  bag  tied  upon  a  hollow  stem  of  wood,  across  which 
rubber  bands  are  stretched,  and  the  vibration  of  which  caused 
by  the  air  within  the  distended  bag  gives  rise  to  the  note, 

It  is  especially  important  to  recognize  the  nature,  extent,  and 


Fig.  431. 


Fig.  432. 


Fig.  431.— Lateral  view  of  laryngeal  muscles  (after  Sappey).  1,  body  of  hyoid  hone; 
2,  vertical  section  of  thyroid  cartilage;  3,  horizontal  section  of  thyroid  cartilage, 
turned  downward  to  show  deep  attachment  of  crico-thyroid  muscle;  4,  facet  of 
the  articulation  of  small  cornu  of  thyroid  cartilage  with  cricoid  cartilage;  5,  facet 
on  cricoid  cartilage;  6,  superior  attachment  of  crico-thyroid  muscle;  7,  posterior 
crico-arytenoid  muscle;  8,  lateral  crico-arytenoid  muscle;  9,  thyro-arytenoid  mus- 
cle; 10,  arytenoid  muscle  proper;  11,  aryteno-epiglottidean  muscle;  12,  middle 
thyro-hyoid  ligament;  13,  lateral  thyro-hyoid  ligament. 

Fig.  432. — Distribution  of  nerves  in  larynx  of  horse  (Chauvean,  after  Toussaint).  a, 
base  of  tongue;  b,  epiglottis;  c,  arytenoid  muscles;  d,  section  of  thyroid  cartilage 
to  show  pails  it,  covers;  e,  cricoid  cartilage; /,  trachea;  g,  (esophagus;  h,  thyro- 
arytenoid muscle;  i,  lateral  crico-arytenoid  muscle;  j,  posterior  crico-arytenoid 
muscle;  k,  arytenoid  muscle;  1,  superior  laryngeal  nerve;  2,  inferior  laryngeal;  3, 
branches  of  superior  laryngeal  passing  to  epiglottis  and  tongue;  4,  branches  of 
superior  laryngeal  passing  to  oesophagus;  5,  very  fine  multiple  anastomoses  be- 
tween two  laryngeals;  fi,  tracheal  branches;  7,  branch  to  posterior  crico-arytenoid 
muscle;  a  portion  is  distributed,  through  the  muscles,  to  subjacent  mucous  mem- 
brane; lo,  branch  passing  to  arytenoid  muscle;  11,  oesophageal  branch  to  aryte- 
noid muscle;  11,  (esophageal  branch  of  pharyngeal  nerve;  it  sometimes  comes 
from  external  laryngeal. 


THE    VOICE. 


595 


effect  on  the  vocal  bands  of  the  movements  of  the  arytenoid 
cartilages.  These  are  most  marked  around  a  vertical  axis,  giv- 
ing rise  to  an  inward  and  outward  movement  of  rotation,  but 


Fig.  433. — Diagrammatic  section  of  larynx  to  illustrate  action  of  Posterior  crico-aryte 
noid  muscle  (after  Landois).  In  this  and  the  two  following  figures  the  dotted 
lines  indicate  the  new  position  of  the  parts  owing  to  the  action  of  the  muscles 
concerned. 

there  are  also  movements  of  less  extent  in  all  directions.     It  is 
in  fact  through  the  movements  of  these  cartilages  to  which  the 


Fir.  434.— Diagrammatic  section  of  larynx  to  illustrate  action  of  Arytenoids*  jirc- 
privs  musbb  (after  Landois). 


596 


COMPARATIVE   PHYSIOLOGY. 


Fig.  435. — Illustrates  action  of  thyro-arytenoideus  interims. 

vocal  bands  are  attached  posteriorly,  that  most  of  the  important 
changes  in  the  tension,  approximation,  etc.,  of  the  latter  are 
produced.  The  lungs  are  to  he  regarded  as  the  bellows  furnish- 
ing the  necessary  wind-power  to  set  the  vocal  bands  vibrating, 
while  the  larynx  has  respiratory  as  well  as  vocal  functions,  as 
has  been  already  learned.  Assuming  that  the  student  has  a 
good  knowledge  of  the  general  anatomy  of  the  larynx,  we  call 
attention  briefly  to  the  following  : 

Widening  of  the  glottis  is  effected  by  the  crico-arytenoideus 
posticus  pulling  outward  the  processus  vocalis  or  attachment 
posteriorly  of  the  vocal  band,  and  a  similar  effect  is  produced 
by  the  arytenoideus  posticus  acting  alone. 

Narrowing  of  the  glottis  is  accomplished  by  the  crico-aryt- 
enoideus lateralis,  and  the  following  when  acting  either  singly 
(except  the  arytenoideus  posticus),  or  in  concert,  as  the  sphinc- 
ter of  the  larynx,  viz.,  the  thyro-arytenoideus  externus,  thyro- 
arytenoideus  internus,  thyro-aryepiglotticus  arytenoideus  pos- 
ticus. 

Tension  of  the  vocal  bands  is  brought  about  by  the  sphincter 
group,  and  especially  by  the  external  and  internal  thyro-aryte- 
noid  muscles. 

Nerve  Supply. — The  superior  laryngeal  contains  the  motor 
fibers  for  the  crico-thyroid  (possibly  also  the  arytenoideus  pos- 
ticus) and  also  supplies  the  mucous  membrane.  The  inferior 
laryngeal  supplies  all  the  other  muscles.  "While  both  of  these 
nerves  are  derived  from  the  vagus,  their  fibers  really  belong  to 
the  spinal  accessory.     It  is  worthy  of  note  that  the  entire  group 


THE   VOICE. 


597 


of  muscles  making  up  the  sphincter  of  the  larynx  is  contracted 
when  the  inferior  laryngeal  is  stimulated. 


Superior  Face.  Inferior  Face. 

Fig.  436.— Cartilaginous  pieces  of  the  larynx  of  horse,  maintained  in  their  natural 
position  by  the  articular  ligaments  (Chauveau).  a,  cricoid  cartilage;  b.  b,  aryte- 
noid cartilages;  c,  body  of  the  thyroid;  c',  c',  lateral  plates  of  the  thyroid;  d.  epi- 
glottis; e,  body  of  the  hyoid;  /,  trachea.  1,  crico-arytenoid  articulation;  2,  capsule 
of  the  crico-thyroid  articulation;  3,  crico-thyroid  membrane;  4,  thyro-hyoid  mem- 
brane; 5,  crico-trachealis  ligament. 


Above  the  true  vocal  bands  composed  of  elastic  fibers  lie  the 
so-called  false  vocal  bands  (cords)  to  be  regarded  as  folds  of  the 
mucous  membrane  which  take  no  essential  part  in  voice-produc- 
tion. Between  these  two  pairs  of  bands  are  the  ventricles  of 
Morgagni,  which,  as  well  as  the  adjacent  parts,  secrete  mucus 
and  allow  of  the  movements  of  both  sets  of  bands  and  in  so  far 
only  assist  in  phonation. 

The  whole  of  the  supra-laryngeal  cavities,  the  trachea  and 
bronchial  tubes,  may  be  regarded  as  resonance-chambers,  the 
former  of  which  are  of  the  most  importance,  so  far  as  the 
quality  of  the  voice  is  concerned.  There  seems  to  be  little 
doubt  that  they  have  much  to  do  with  determining  the  differ- 
ences by  which  one  individual's  voice  at  the  same  pitch  differs 
from  another ;  nor  is  the  view  that  they  may  have  a  slight  in- 
fluence on  the  pitch  of  the  voice,  or  even  its  intensity,  to  be 
ignored. 


'598 


COMPARATIVE   PHYSIOLOGY. 


The  epiglottis,  in  so  far  as  it  has  any  effect,  in  all  probability 
modifies  the  voice  in  the  direction  of  quality. 

Pathological.— Paralysis  of 
the  laryngeal  muscles,  owing  to 
pressure  on  nerves  and  conse- 
quent narrowing  of  the  glottic 
opening,  explains  "  roaring  "  in 
the  horse,  in  certain  instances 
at  all  events. 

Comparative.— Much  more 
is  known  of  the  sounds  emanat- 
ing from  the  lower  animals 
than  of  the  mechanisms  by 
which  they  are  produced.  This 
applies,  of  course,  especially  to 
such  sounds  as  are  not  pro- 
duced by  external  parts  of  the 
body,  it  being  very  difficult  to 
investigate  these  experimental- 
ly or  to  observe  the  animal 
closely  enough  when  produc- 
ing the  various  vocal  effects 
naturally. 

All  our  domestic  mammals 
have  a  larynx,  not  as  widely  different  from  that  of  man  as 
might  be  supposed  from  the  feeble  range  of  their  vocal  powers. 
There  are  structural  differences  in  the  larynx  of  the  domestic 
animals,  some  of  which  are  more  readily  appreciated  by  the  eye 
than  described. 

The  false  (superior)  vocal  bands  are  rudimentary  or  want- 
ing in  many  mammals,  including  the  horse,  ass,  etc. 

In  ruminants  the  larynx  is  proportionately  ill-developed ; 
the  glottis  is  short,  the  vocal  bands  rudimentary,  and  the  ven- 
tricles wanting. 

The  larnyx  of  the  pig  is  peculiar  in  that  the  ventricles  are 
deep,  though  their  opening  is  only  a  narrow  slit;  there  is,  how- 
ever, a  large  membranous  sac  below  the  epiglottis,  which, 
acting  as  a  resonator,  explains  the  great  intensity  of  the  voice 
of  this  animal. 

The  actual  behavior  of  the  vocal  bands  has  been  studied 
experimentally,  in  the  dog  when  growling,  barking,  etc.  And, 
so  far  as  it  goes,  this  animal's  mechanism  of  voice-production 


Fig.  437.—  Posterolateral  view  of  the  lar- 
ynx of  the  horse  (Chauveau).  1,  epi- 
glottis ;  2,  arytenoid  cartilages  ;  3, 
thyroid  cartilage ;  4,  arytenoideus 
muscle;  5,  crico-arytenoideus  latera- 
lis; 6,  thyro-arytenoideus;  7,  crico- 
arytenoideus  posticus:  8,  crico-thy- 
roidens;  9,  ligament  between  the  cri- 
coid cartilage  and  first  ring  of  trachea; 
10.  11,  infero-posterior  extremities  of 
crico-thyroid  cartilages. 


THE   VOICE. 


599 


is  not  essentially  different  from  that  of  man.  Growling  is  the 
result  of  a  functional  activity  of  the  vocal  mechanism,  not  un- 
like that  of  man  when  singing  a  bass  note;  barking,  of  that 
analogous  to  coughing  or  laughing,  when  the  vocal  bands  are 
rapidly  approximated  and  separated. 

The  grunting  of  hogs  and  the  lowing  and  bawling  of  horned 
cattle  are  probably  very  similar  in  production,  so  far  as  the 
larynx  is  concerned,  to  the  above.  The  cat  has  plainly  very 
great  command  over  the  larynx,  and  can  produce  a  wide  range 
of  tones.  The  peculiarities  of  the  bray  of  the  ass  are  owing  to 
voice  production  both  during  inspiration  and  expiration. 

The  quality  of  the  voice  of 
most  animals  appears  harsh  to 
our  ears,  owing  probably  "to  a 
great  preponderance  of  over- 
tones, in  consequence  of  an  im- 
perfect and  unequal  tension  of 
the  vocal  bands;  but  the  influ- 
ence of  the  supra-laryngeal  cavi- 
ties, often  very  large,  must  also  be 
taken  into  account. 

In  certain  of  the  primates,  and 
especially  in  the  howling  mon- 
keys, large  cheek-pouches  can  be  Fig.  438-Lower  larynx  (Syrinx)  of 
inflated  with  air  from  the  larynx, 
and  so  add  to  the  intensity  of  the 
note  produced  by  the  vocal  bands 
that  then*  voice  may  be  heard  for 
miles.  Song-birds  produce  their 
notes,  as  may  be  seen,  by  exter- 
nal movements  low  down  at  the  bifurcation  of  the  trachea 
(syrinx).  The  notes  are  owing  to  the  vibration  of  two  folds  of 
the  mucous  membrane,  which  project  into  each  bronchus,  and 
are  regulated  in  their  movements  by  muscles,  the  bronchial 
rings  in  this  region  being  correspondingly  modified. 

A  large  number  of  species  of  fishes  produce  sounds  and  in 
a  variety  of  ways,  in  which  the  air-bladder,  stomach,  intestines. 
etc.,  take  part.  Most  reptiles  are  voiceless,  in  the  proper  sense, 
though  there  are  few  that  can  not  produce  a  sort  of  hissing 
sound,  caused  by  the  forcible  emission  of  air  through  the  upper 
respiratory  passages. 

Frogs,  as  is  well  known,  produce  sounds  of  great  variety  in 


crow  (after  Gegenbaurl."  A,  seen 
from  side:  B,  seen  from  in  front. 
a—f,  muscles  concerned  in  move- 
ments of  lower  larynx;  rj.  mem- 
brana  tympaniformis  interna, 
stretching  from  median  surface  of 
either  bronchus  to  a  bony  ridge 
(.pessulus)  which  projects  at  the 
angle  of  bifurcation  of  trachea. 


(500  COMPARATIVE   PHYSIOLOGY. 

pitch,  quality,  and  intensity,  some  species  croaking  so  as  to  be 
heard  at  the  distance  of  at  least  a  mile.  It  is  a  matter  of  easy 
observation  that  when  frogs  croak  the  capacity  of  the  mouth 
cavity  is  greatly  increased,  owing  to  the  distention  of  resonat- 
ing sacs  situated  at  each  angle  of  the  jaws.  When  tree-frogs 
croak,  their  throats  are  greatly  distended,  apparently  in  suc- 
cessive waves. 

SPECIAL   CONSIDERATIONS   AND    SUMMARY. 

Evolution. — The  very  lowest  forms,  and  in  fact  most  inverte- 
brate groups,  seem  to  be  voiceless.  Darwin  has  shown  that 
voice  is,  in  a  large  number  of  groups,  confined  either  entirely 
to  the  male,  or  that  it  is  so  much  more  developed  in  him  as  to 
become  what  he  terms  a  "  sexual  character."  There  is  abundant 
evidence  that  males  are  chosen  as  mates  by  the  females,  among 
birds  especially,  not  alone  for  superiority  in  beauty  of  plumage, 
but  also  for  their  song.  Thus,  by  a  process  of  natural  selection 
(sexual  selection),  the  voice  would  tend  to  improve  with  the 
lapse  of  time,  if  we  admit  heredity,  which  is  an  undeniable  fact, 
even  among  men — whole  families  for  generations,  as  the  Bachs, 
having  been  musicians. 

One  can  also  understand  why  on  these  principles  voice 
should  be  especially  developed  in  certain  groups  (birds),  while 
among  others  (mammals)  form  and  strength  should  determine 
sexual  selection,  the  strongest  winning  in  the  contests  for  the 
possession  of  the  females,  and  so  propagating  their  species  under 
the  more  favorable  circumstance  of  choice  of  the  most  desira- 
ble females. 

Pathology  teaches  that,  when  certain  parts  of  the  brain 
(speech-centers)  of  man  are  injured  by  accident  or  disease,  the 
power  of  speech  may  be  lost.  From  this  it  is  evident  that  the 
vocal  appai-atus  may  be  perfect  and  yet  speech  be  wanting;  so 
that  it  becomes  comprehensible  that  the  vocal  powers  of,  e.  g., 
a  dog,  are  so  limited,  notwithstanding  his  comparatively  highly 
developed  larynx.  He  lacks  the  energizing  and  directive  ma- 
chinery situated  in  the  brain. 

Some  believe  that  there  was  a  period  when  man  did  not  pos- 
sess the  power  of  speech  at  all ;  and  many  are  convinced  that 
the  human  race  have  undergone  a  gradual  development  in  this 
as  in  other  respects.  Certain  it  is  that  races  differ  still  very 
widely  in  capacity  to  express  ideas  by  spoken  words. 


THE   VOICE  601 

We  may  regard  the  development  of  a  race  of  speaking  ani- 
mals as  dependent  upon  a  corresponding  advance  in  brain- 
structure,  whether  that  was  acquired  by  a  sudden  and  pro- 
nounced variation,  or  by  gradual  additions  of  increase  in  cer- 
tain regions  of  the  brain,  or  whether  to  the  first  there  was  then 
added  the  second. 

Apart  from  speech  proper,  there  is  a  language  of  the  face 
and  body  generally,  in  which  there  is  much  that  we  share  with 
lower  forms,  especially  lower  mammals.  Darwin,  noticing  this 
resemblance,  regarded  it  as  evidence  strengthening  the  belief 
that  man  is  derived  from  lower  forms.  Why  should  the  forms 
of  facial  expression  associated  so  generally  with  certain  emotions 
among  different  races  of  men  be  so  similar  to  each  other  and 
to  those  which  the  lower  animals  employ,  if  there  is  not  some 
community  of  origin  ?  This  is  Darwin's  query,  and  he  con- 
sidered, as  has  been  stated,  that  the  answer  to  be  given  was  in 
harmony  with  his  views  of  man's  origin,  as  based  on  an  alto- 
gether different  sort  of  testimony. 

The  high  functional  development  of  the  hand  and  arm  in 
man,  and  the  use  of  these  parts  in  writing,  are  suggestive. 

Summary. — The  musical  tones  of  the  voice  are  caused  by  the 
vibrations  of  the  vocal  bands,  owing  to  the  action  on  them  of 
an  expiratory  blast  of  air  from  the  lungs.  In  order  that  the 
bands  may  act  effectively,  they  must  be  rendered  tense  and  ap- 
proximated, which  is  accomplished  by  the  action  of  the  laryn- 
geal muscles,  especially  those  attached  to  the  arytenoid  carti- 
lages. We  may  speak  of  the  respiratory  glottis  and  the  vocal- 
izing glottis,  according  as  we  consider  the  position  and  move- 
ments of  the  vocal  bands  in  respiration  or  in  phonation. 

The  pitch  of  the  voice  is  determined  by  the  length  and  the 
tension  of  the  vocal  bands,  and  frequently  both  shortening  and 
increased  tension  are  combined;  perhaps  we  may  say  that  al- 
tered (not  necessarily  increased)  tension  and  length  are  always 
combined. 

The  quality  of  the  voice  depends  chiefly  upon  the  supi-a- 
laryngeal  cavities. 

It  is  important  to  remember  that  in  all  phonation,  in  the 
case  of  man  at  least,  many  parts  combine  to  produce  the  result: 
so  that  voice-production  is  complex  and  variable  in  mechanism, 
beyond  what  would  be  inferred  from  the  apparent  simplicity  of 
the  mechanism  involved ;  while  the  central  nervous  processes 
are,  when  comparison  is  made  with  phonation  in  lower  ani- 


602  COMPARATIVE   PHYSIOLOGY. 

mals,  seen  to  be  the  most  involved  and  important  of  the  whole 
— a  fact  which  the  results  of  disease  of  the  brain  are  well  calcu- 
lated to  impress,  inasmuch  as  interruptions  anywhere  among  a 
class  of  cerebral  connections,  now  known  to  be  very  extensive, 
suffice  to  abolish  voice,  and  especially  speech -production. 

Among  mammals  below  man  the  vocal  bands  and  laryngeal 
and  thoracic  mechanism  are  very  similar,  but  less  perfectly 
and  complexly  co-ordinated ;  so  that  their  vocalization  is  more 
limited  in  range,  and  their  tones  characterized  by  a  quality 
which  to  the  human  ear  is  less  agreeable.  Man's  superiority  as 
a  speaking  animal  is  to  be  traced  chiefly  to  the  special  develop- 
ment of  his  cerebrum,  both  generally  and  in  certain  definite 
regions. 


CERTAIN   TISSUES. 


Prior  to  considering  the  subject  of  the  next  chapter,  it  may 
be  well  to  give  a  short  account  of  certain  tissues  specially  con- 
cerned. 

Connective  Tissue.— This  is  the  most  widely  distributed 
tissue  in  the  body,  since  it  binds  together  all  other  forms  of 
tissue,  and,  in  some  of  its  many  varieties,  enters  into  the  forma- 
tion of  every  organ.  As  connective  tissue  proper,  its  function 
is  subordinate ;  but  when  it  becomes  the  aponeuroses  of  mus- 


Fig.  439.— Fibers  of  tendon  of  man  (Rollettt. 


cles,  and  especially  tendons,  by  which,  from  its  inextensibility, 
the  muscles  are  rendered  effective  in  moving  the  levers  (bones) 
to  which  they  are  attached,  its  importance  is  more  pronounced. 
In  structure,  this  fibrous  tissue  consists  of  bundles  of  fine  fibrils, 
among  which,  especially  in  the  younger  stage,  connective-tissue 
cells  may  be  found,  and  from  which  the  Abel's  themselves  are 
formed. 


604 


COMPARATIVE   PHYSIOLOGY. 


It  is  owing-  to  differences  in  the  shape  and  size  of  these  cells 
chiefly  that  the  structural  variations  of  connective  tissue  in  dif- 
ferent regions  of  the  body  are  due. 


1  1 

I 


Fig.  440.— Loose  network  of  connective  tissue  from  man,  in  which  are  connective-tis- 
sue corpuscles  among  the  fibers  (Rollet).    a,  a,  capillary  with  blood-cells. 

Elastic  Tissue. — This  form  of  tissue  is  also  of  very  wide  distri- 
bution and  of  great  importance  in  the  economy  of  a  complicated 
living  organism  that  must  constantly  adapt  itself  to  the  stress 
and  strains  of  existence.  In  its  purest  form  it  occurs,  e.  g.,  in 
the  ligamentum  nuclese  of  the  ox,  as  a  somewhat  yellow,  tough, 
elastic  structure  easily  fibrillated  when  boiled,  but  with  diffi- 
culty torn  asunder  when  fresh.  Under  the  microscope  it  ap- 
pears as  fibers  with  a  very  distinct  outline  and  of  varying  size. 
In  the  arteries,  as  already  referred  to,  it  forms  a  sort  of  elastic 
membrane  of  the  utmost  importance  in  the  functions  of  these 
organs. 

Bone. — In  a  long  bone,  as  the  femur,  in  the  dried  state,  we 
recognize  a  compact  shaft  and  two  extremities  of  a  more  porous 
nature,  while  the  central  portion  of  the  former  presents  a  more 
or  less  circular  cavity,  the  medullary  canal.  By  treatment 
with  hydrochloric  acid  abundance  of  lime  salts  may  be  ob- 


CERTAIN    TISSUES. 


605 


^■^{^"y.n.;  k 


Fig.  433. 


Fig.  441. 


Fig.  442. 


Fig.  441. — Fine  elastic  fibers  from  peritonaeum,  1  x  350  (Kolliker). 
Fig.  442. — Larger  elastic  fibers  (Robin). 

Fig.  443. — Elastic  network  (fenestrated  membrane)  from  middle  coat  of  carotid  of 
horse,  1  x  350  (Kolliker). 


ym^miMm  ill* 


8 


n  ip**;*tsiv, 


':  ty\W 


Fig.  444.— Longitudinal  section  of  humerus,  showing  Haversian  canals  and  lacunre, 
1  x  200  (Sappey). 


606 


COMPARATIVE   PHYSIOLOGY. 


Fig.  445. — Transverse  section  of  humerus.  1  x  200  (Sappey).  1,  section  of  vascular 
(Haversian)  cells;  2,  longitudinal  canal  at  point  of  junction  with  transverse  canal. 
Lacunae  and  canaliculi  arranged  in  concentric  rings. 

tained.     A  microscopic  transverse  section  shows  the  substance 
of  the  shaft  to  be  penetrated  by  longitudinal  channels  (Haver- 


r  r> 


Pig.  446. — Bone-corpuscles  and  their  processes  which  fill  the  lacunae  and  canaliculi 
(Rollett). 


CERTAIN    TISSUES. 


601 


sian  canals),  while  the  intermediate  space  is  occupied  by  cavi- 
ties (lacunae)  connected  with  one  another  by  very  fine  canals 


thl 


Fig.  447. — Vertical  section  of  articular  cartilage  resting  on  bone,  and  showing  cells 
and  capsules  arranged  in  layers  as  indicated  by  numerals  (Sappey). 

(Fig.  444).  A  vertical  cross  -  section  exhibits  the  lamella?  of 
which  it  is  made  up  and  the  vascular  channels  cut  across 
(Fig.  445). 

All  this  is,  however,  only  the  framework  of  osseous  tissue. 
If  a  bone  from  an  animal  freshly  killed,  without  bleeding,  be 
examined,  a  very  different  state  of  things  will  be  found.  The 
bone  is  heavier ;  its  surface  is  covered  with  a  closely  adherent, 
tough,  fibrous  structure,  the  periosteum  :  and  its  medullary 
cavity  filled  with  marrow.  If  the  bone  be  broken  across,  its 
section  looks  red,  and  blood  flows  from  the  surface.  Investiga- 
tion proves  that  the  covering  periosteum  is  a  bed  in  which 
blood-vessels  and  nerves  ramify,  and  from  which  they  enter 


608 


COMPARATIVE   PHYSIOLOGY- 


the  openings  to  be  seen  on  the  surface  of  the  dead  bone.  The 
Haversian  canals  are  vascular  channels,  and  the  lacunae  filled 
with  bone  corpuscles  (Fig.  446).  The  marrow  in  the  extremi- 
ties of  the  bone  is  of  a  red  color  in  consequence  of  its  great  vas- 
cularity; and  in  the  young  animal  a  similar  marrow  fills  the 


Pia.  448.— Section  of  cartilage  of  ear  of  man  (Rollett).    a,  fibre-cartilage;  b,  connect- 
ive tissue.    The  cartilage  had  been  boiled  and  dried  prior  to  cutting. 

medullary  canal,  but  later  it  is  less  vascular,  and  abounds  in 
fat.     Blood-vessels  pass  from  it  into  the  compact  tissue  of  the 


CERTAIN  TISSUES.  609 

bone.  The  main  artery,  whence  the  others  are  derived,  for  the 
shaft  of  the  hone,  enters  by  the  nutrient  foramen  on  the  sur- 
face, and  toward  the  center. 

The  bone-corpuscles  (Fig.  446),  answering  to  the  connective- 
tissue  cells,  are  nutritive  and  formative  after  a  considerable 
portion  of  the  tissue  has  become  the  seat  of  the  deposit  of  lime- 
salts.  Bone  is  a  living  tissue,  though  in  a  less  degree  than 
most  others ;  but  it  is  only  by  bearing  these  relations  in  mind 
that  its  function  in  the  support  of  the  soft  parts  of  an  animal, 
and  especially  as  constituting  the  essential  levers  of  its  locomo- 
tive mechanism,  can  be  understood. 

Cartilage. — In  the  earliest  stages  of  an  animal's  existence 
the  bones  are  represented  by  cartilage,  and  at  all  periods  of  its 
existence  this  structure  forms  those  elastic  pads  that  cover  its 
articular  surfaces,  and  shield  the  bones  and  the  entire  animal 
from,  undue  concussion.  The  kind  of  cartilage  that  covers  the 
extremities  of  the  long  bones,  known  as  articular,  is  character- 
ized by  abundance  of  cells  lying  in  a  homogeneous  bed  or  ma- 
trix (Fig.  447). 

Fibro-cartilage  (Fig.   448)  abounds  in  fibrous  tissue,  some 
elastic  fibers,  characteristic  cells,  etc.,  and  is  found  between  the 
bodies  of  the  vertebrae  and  in  similar  situations,  as  well  as  in  the 
epiglottis,  the  ear,  etc. 
39 


LOCOMOTION. 


The  entire  locomotor  system  of  tissues  is  derived  from  the 
embryonic  mesoblast.  These  include  the  muscles,  hones,  carti- 
lage, and  connective  and  fibrous  tissues  ;  and  the  tissues  that 
make  up  the  vascular  system  or  the  motor  apparatus  for  the 
circulation  of  the  blood.  Locomotion  in  the  mammal  is  effected 
by  the  movement  of  certain  bony  levers,  while  the  equilibrium 

of  the  body  is  maintained. 
The  whole  series  of  levers  is 
bound  together  by  muscles, 
tendons,  ligaments,  etc.,  and 
play  over  one  another  at  cer- 
tain points  where  they  are  in- 
vested with  cartilage,  and 
kept  moist  by  a  secretion  from 
the  cells  covering  the  syno- 
vial membranes  that  form  the 
inner  linings  of  joints. 

Cartilage,  a  very  low  form 
of  tissue  destitute  of  blood- 
vessels, and  hence  badly  re- 
paired when  lost  by  injury 
or  disease,  forms  a  series  of 
smooth  surfaces  admirably 
adapted  for  joints,  and  espe- 
cially fitted  to  act  as  a  series 
of  elastic  buffers,  and  thus 
prevent  shocks.  Bone,  though 
brittle  in  the  dried  state,  possesses,  when  alive,  a  favorable  de- 
gree of  elasticity,  while  sufficiently  rigid.  Provision  is  made 
by  its  vascular  periosteum  and  central  marrow  (in  the  case  of 
the  long  bones),  as  Avell  as  by  the  blood-supply  derived  from 
the  nutrient  artery  and  its  ramifications  throughout  the  osseous 


LOCOMOTION. 


611 


tissue,  for  abundant  nourishment,  growth,  and  repair  after  in- 
jury- 

We  find  in  the  body  of  mammals,  including  man,  examples 

of  all  three  kinds  of  levers.  It  sometimes  happens  that  there 
is  an  apparent  sacrifice  of  energy,  the  best  leverage  not  being 
exemplified  ;  but  on  closer  examination  it  will  be  seen  that  the 
weight  must  either  be  moved  with  nice  precision  or  through 
large  distances,  and  these  objects  can  not  be  accomplished  al- 
ways by  the  arrangements  that  would  simply  furnish  the  most 
powerful  lever.  This  is  illustrated  by  the  action  of  the  biceps 
on  the  forearm. 

It  is  to  be  remembered  that,  while  the  flexors  and  extensors 
of  a  limb  act  in  a  certain  degree  the  opposite  of  one  another, 


Fig.  452. — Skeleton  of  deer.  The  bone.*  in  the  extremities  of  this  the  fleetest  of  quad- 
rupeds are  inclined  very  obliquely  toward  each  other  and  toward  the  scapular  and 
iliac  bones.  This  arrangement  increases  the  leverage  of  the  muscular  system  and 
confers  great  rapidity  on  the  moving  parts.  It  augments  elasticity,  diminishes 
shock,  and  indirectly  begets  continuity  of  movement,  n.  angle  formed  by  femur 
with  ilinm;  6,  angle  formed  by  tibia  and  fibula  with  femur;  c,  angle  formed  by 
phalanges  with  cannon-bone;  e,  angle  formed  by  humerus  with  scapula:  /.  angle 
formed  by  radius  and  ulna  with  humerus  (Pettigrew). 

there  is  also,  in  all  cases  perhaps,  a  united  action  ;  the  one 
set,  however,  preponderating  over  the  other,  and  usually  sev- 
eral muscles,  whether  of  the  same  or  different  classes,  act  to- 
gether. 

Standing  itself  requires  the  exercise  of  a  large  number  of 


612 


COMPARATIVE   PHYSIOLOGY. 


similar  and  antagonistic  muscles  so  co-ordinated  that  the  line 
of  gravity  falls  within  the  area  of  the  feet.  An  unconscious 
animal  falls,  which  is  itself  an  evidence  of  the  truth  of  the 
above  remarks. 

The  folio-wing  statements  in  regard  to  the  direction  of  the 
line  of  gravity  in  man  may  prove  useful  :  1.  That  for  the  head 
falls  in  front  of  the  occipital  articulation,  as  exemplified  by  the 
nodding  of  the  head  in  a  drowsy  person  occupying  the  sitting 
attitude.  2.  That  for  the  head  and  trunk  together  passes  behind 
a  line  joining  the  centers  of  the  two  hip-joints,  hence  the  uncor- 
rected tendency  of  the  erect  body  of  man  is  to  fall  backward. 
3.  That  for  the  head,  trunk,  and  thighs  falls  behind  the  knee- 
joints  somewhat,  which  would  also  favor  falling  backward 
(bending  of  the  knees).  4.  The  line  of  gravity  of  the  whole 
body  passes  in  front  of  a  line  joining  the  two  ankle-joints,  so 


Fig.  453.— Shows  the  simultaneous  positions  of  both  legs  during  a  step,  divided  into 
four  groups  (after  Weber).  First  group  (,4),  4  to  7,  gives  the  different  positions 
which  the  legs  simultaneously  assume  while  both  are  on  the  ground;  second  group 
(B),  8  to  11,  shows  the  various  positions  of  both  legs  at  the  time  when  the  poste- 
rior leg  is  elevated  from  the  ground,  but  behind  the  supported  one;  third  group 
(C),  12  to  14,  shows  the  positions  which  the  legs  assume  when  the  swinging  leg 
overtakes  the  standing  one;  and  the  fourth  group  (/)),  1  to  3.  the  positions  during 
the  time  when  the  swinging  leg  is  propelled  in  advance  of  the  resting  one.  The 
letters  it.  h,  and  c  indicate  the  angles  formed  by  (he  bones  of  the  right  leg  when 
engaged  in  making  a  step;  the  letters  m,  n,  and  o,  the  positions  assumed  by  the 
right  foot  when  the  trunk  is  rolling  over  it;  {/,  shows  the  rotating  forward  of  the 
trunk  upon  the  left  foot  (/)  as  an  axis;  h,  shows  the  rotating  forward  of  the  left 
leg  and  foot  upon  the  trunk  ((/)  as  an  axis. 


that  the  body  would  tend,  but  for  the  contraction  of  the  mus- 
cles of  the  calves  of  the  legs,  to  fall  forward. 

Taking  these  different  facts  into  consideration  explains  the 


LOCOMOTION. 


013 


various  directions  in  which  an  individual,  when  erect,  may  fall 
according1  as  one  or  the  other  line  (centei'j  of  gravity  is  dis- 
placed for  a  long  enough  time. 

Walking  (man)  implies  the  alternate  movement  of  each  leg 
forward,  pendulum-like,  so  that  for  a  moment  the  entire  body 
must  be  supported  on  one  foot.  "When  the  right  foot  is  lifted 
or  swung  forward,  the  left  must  support  the  weight  of  the 
body.  It  becomes  oblique,  the  heel  being  raised,  the  toe  still 
resting  on  the  ground  ;  and  it  is  upon  this  as  a  fulcrum  that 
the  body -weight  is  moved  forward,  when  a  similar  action  is 
taken  up  by  the  opposite  leg. 

It  follows  that  to  prevent  a  fall  there  must  be  a  leaning  of 
the  body  to  one  side,  so  that  the  line  of  gravity  may  pass  through 
each  stationary  foot ;  hence  a  person  walking  describes  a  series 
of  vertical  curves  with  the  head  and  of  horizontal  ones  with  the 
body,  the  resulting  total  being  complex. 


Fig?.  -154  and  455. — Showing  the  more  or  less  perpendicular  direction  of  the  stroke  of 
the  wing  in  the  flight  of  the  bird  (gull);  how  the  wing  is  gradually  extended  as  it 
is  elevated  (e,f,  g)\  how  it  descends  as  a  long  lever  until  it  assumes  the  position 
indicated  by  A :  how  it  is  flexed  toward  thetermination  of  the  down-stroke,  as 
shown  at  h'i.j.  to  convert  it  into  a  short  lever  («.  ti)  and  prepare  it  for  making  the 
lip-Stroke.  The  difference  in  the  length  of  the  w  ing  during  flexion  and  extension 
is  indicated  by  the  short  and  Jong  levers  a,  b  and  c.  d.  The  sudden  conversion  of 
the  wing  froni  a  Ion;;  into  a  short  lever  at  the  end  of  the  down-stroke  is  of  great 
importance,  as  it  robs  the  wing  of  its  momentum  and  prepares  it  for  reversing  its 
movements  (Pettigrew). 

The  peculiarities  of  the  gait  of  different  persons  are  naturally 
determined  by  their  height,  length  of  leg.  and  a  variety  of  other 
factors,  which  are  often  inherited  with  great  exactness.  We 
instinctively  adopt  that  gait  which  economizes  energy,  both 
physical  and  mental. 

Running  differs  from  walking,  in  that  both  feet  are  for  a 


614 


COMPARATIVE  PHYSIOLOGY. 


Figs.  450  and  457  show  that  when  the  wings  are  elevated  («,/,  g)  the  body  falls  («); 
and  that  when  the  wings  are  depressed  (h,  i,j)  the  body  is  elevated  (r).  Fig.  456 
shows  that  the  wings  are  elevated  as  short  levers  (e)  until  toward  the  termination 
of  the  up-stroke,  when  they  are  gradually  expanded  (/,  g)  to  prepare  them  for 
making  the  down-stroke.  Fig.  457  shows  that  the  wings 'descend  as  long  levers 
(A)  until  toward  the  termination  of  the  down-stroke,  when  they  are  gradually 
folded  or  flexed  (i,j)  to  rob  them  of  their  momentum  and  prepare  them  for  mak- 
ing the  up-stroke.  (Compare  with  Figs.  454  and  455.)  By  this  means  the  air  be- 
neath the  wings  is  vigorously  seized  during  the  down-stroke,  while  that  above  it 
is  avoided  during  the  up-stroke.  The  concavo-convex  form  of  the  wings  and  the 
forward  travel  of  the  body  contributes  to  this  result.  The  wings,  it  will  be  ob- 
served, act  as  a  parachute  both  during  the  up  and  down  strokes.  Fig.  457  shows 
also  the  compound  rotation  of  the  wing,  how  it  rotates  upon  a,  as  a  center,  with 
a  radius  m,  b,  n,  and  upon  a,  c,  b  as  a  center,  with  a  radius  k,  I  (Pettigrew). 

period  of  the  cycle  off  the  ground  at  the  same  time,  owing  to  a 
very  energetic  action  of  the  foot  acting  as  a  fulcrum. 

Jumping  implies  the  propulsion  of  the  body  by  the  impulse 
given  by  both  feet  at  the  same  moment. 

Hopping  is  the  same  act  accomplished  by  the  use  of  one 
leg. 

Comparative. — Tbe  movements  of  quadrupeds  are  naturally 
very  complicated,  but  have  now  been  well  worked  out  by  the 
use  of  instantaneous  photography.  Even  the  bird's  flight  is  no 
longer  a  wholly  unsolved  problem,  but  is  fairly  well  under- 
stood. The  movements  of  centipedes  and  and  other  many- 
legged  invertebrates  are  highly  complicated,  while  their  rapid 
movements  are  to  be  accounted  for  by  the  multiplicity  of  their 
levers  rather  than  the  rapidity  with  which  they  are  moved. 


LOCOMOTION.  615 

The  length  and  flexibility  of  their  bodies  must  also  be  taken 
into  account,  rendering1  many  legs  necessary  for  support. 

The  subject  of  locomotion  is  of  such  great  importance  in  the 
practice  of  comparative  medicine  that  we  shall  now  enter  upon 
it  in  somewhat  more  detail,  especially  as  regards  the  horse. 
This,  of  all  our  domestic  animals,  has  become  specialized  as  a 
locomotive  mechanism.  All  the  parts  of  his  whole  economy 
have  been  co-ordinated  to  that  end ;  and,  except  the  horse  be 
viewed  in  this  light,  the  significance  of  much  in  his  nature 


sJfe^- 


r  — 


Fig.  458.— Chillingham  bull  (Bos  Scoticus).  Shows  powerful,  heavy  body,  and  the 
small  extremities  adapted  for  land  transit.  Also  the  figure-of-8  movements  made 
by  the  feet  and  limbs  in  walking  and  running.  ?/,  t.  curves  made  by  right  and 
left  anterior  extremities;  r,  s,  curves  made  by  right  and  left  posterior  extremities. 
The  right  fore  and  the  left  hind  foot  move  together  to  form  the  waved  line  (s,  w); 
the  left  fore  and  the  right  hind  foot  move  together  to  form  the  waved  line  (/•,  t). 
The  curves  formed  by  the  anterior  (t,  u)  and  posterior  (,/■,  s)  extremities  form  ellipses 
(Pettigrew;. 

will  be  missed.  But,  however  well  his  other  parts  might  be 
suited  to  tbis  purpose,  unless  the  feet  were  adapted  to  rapid 
movements  and  great  and  frequently  repeated  concussions,  the 
animal  must  soon  break  down.  As  it  is,  under  the  unnatural 
conditions  of  our  artificially  constructed  roads,  faulty  shoeing, 
housing,  and  feeding,  lamenesses  of  the  feet  constitute  a  large 
proportion  of  the  cases  that  fall  under  the  care  of  the  practitioner. 
It  may  be  well  at  the  outset  to  give  a  little  consideration  to  the 
feet  of  the  horse,  in  order  to  learn  to  what  extent  they  are 
adapted  to  natural  conditions.  The  feet  of  all  mammals  illustrate 
how  the  soft  and  yielding  tissues  are  combined  with  the  rigid, 
to  adapt  to  conditions  of  the  surface  over  which  they  are  re- 
quired to  move.  In  the  carnivora,  beneath  the  outer  tough  skin 
covering  the  sole,  there  is  the  fatty  cushion  protective  to  the 
bones  and  more  delicate  soft  parts  ;  while  the  claws,  nails,  etc. 


616 


COMPARATIVE   PHYSIOLOGY. 


in  which  the  toes  end,  are  not  only  weapons  of  offense  and  de- 
fense, but  protective  against  injury  from  contact  with  hard  sur- 
faces, as  well  as  directly  helpful  in  locomotion.     These  princ 
pies  are  admirably  exemplified  in  the  foot  of  solipeds. 

The  foot  of  the  horse  may  be  said  to  consist  of  terminal 
bones  incased  in  soft  structures  adapted  to  shield  the  animal 
from  the  effects  of  excessive  concussion  and  for  nutrition,  the 


Fig.  459. 


Fig.  460. 


Fig.  430.— Longitudinal  median  section  of  foot.  1.  anterior  extensor  of  phalanges, 
or  extensor  pedis;  2,  lateral  extensor,  or  extensor  suffraginis;  3,  capsule  of  meta- 
carpophalangeal articulation;  4.  large  metacarpal  bone;  5.  superficial  flexor  of 
phalanges,  or  perforatns;  0.  deep  flexor,  or  perforans;  7,  sheath;  8,  bursa;  9,  sesa- 
moid bone;  10,  ergot  and  fatty  cushion  of  fetlock;  11,  crucial  ligament;  12,  short 
sesamoid  ligament;  13.  first  phalanx;  14,  bursa;  15,  second  phalanx;  16,  navicu- 
lar bone;  17.  plantar  cushion;  18,  third  phalanx;  19,  plantar  surface  of  hoof ;  20, 
sensitive  or  keratosrenous  membrane  of  third  phalanx. 

Fig.  460.— Horizontal  section  of  horse's  foot.  1.  front  or  toe  of  hoof;  2,  thickness  of 
wall:  3.  laminae;  4,  insertion  of  extensor  pedis;  5,  os  pedis;  6,  navicular  bone;  7, 
u -JiiL's  of  os  pedis;  8,  lateral  cartilage;  9,  flexor  pedis  tendon;  10,  plantar  cushion; 
1 1 ,  inflexion  of  wall  or  "  bar  ";  12,  horny  frog. 


whole  being  incased  in  a  protective  covering  which  in  a  state 
of  nature  is  constantly  being  worn  away  and  renewed.  The 
hoof  is  the  homologue  of  the  nails  and  claws  of  other  mammals, 
and  so  may  be  regarded  as  a  modification  of  the  epidermis ; 
and  thus  viewed,  its  structure  is  at  once* more  readily  under- 
stood and  more  interesting.  To  speak  from  an  anatomical 
standpoint,  the  foot  of  the  horse  is  made  up  of  the  terminal 


LOCOMOTION. 


617 


i|\ 


Fig.  461. 

Fig.  461.— Lower  face  of  horse's  foot,  hoof  being  removed.  1,  heel;  2,  coronary  cush- 
ion; 3,  branch  of  plantar  cushion;  4,  median  lacuna;  5,  lamina  of  the  bars;  6, 
velvety  tissue  of  sole. 

Pig.  462.— Lateral  view  of  horse's  foot  after  removal  of  hoof.  1,  perioplic  ring,  divided 
by  a  narrow  groove  from  coronary  cushion,  2,  which  is  continuous  with  plantar 
cushion,  4,  and  joins  vascular  lamina,  3,  through  medium  of  white  zone. 


phalanx,  the  navicular  bone,  and  the  lower  part  of  the  second 
phalanx;  certain  ligaments  entering  into  the  articulations  ;  the 


Fig.  464. 


Fig.  463.— Hoof  just  removed  from  foot;  side  view.  a.  inner  surface  of  periople,  or 
coronary  frog-band,  with  some  hairs  passing  through:  «'.  outer  surface  of  same 
at  posterior  part  of  foot;  a",  a  section  through  the  wall  to  show  its  thickness;  b 
toe,  quarter  of  hoof;  from  b  to  front  is  outside  (or  inside)  toe,  from  c  to  d  the 
outside  (or  inside)  heel;  e,  frog;  /,  bevel,  or  upper  niargiu  of  wall  for  reception  of 
coronary  cushion;  g,  keraphylla,  or  horny  laminae. 

Fig.  464.— Hoof,  with  outer  portion  of  wall  removed  to  show  its  interior,  a,  a,  peri- 
ople, or  coronary  frog-band;  b,  cavity  in  upper  part  of  wall  for  coronary  cush- 
ion; c,  upper  or 'inner  surface  of  •'liar";  d,  vertical  section  of  wall:  </'.  same,  at 
heel;  e,  horizontal  section  of  ditto;  /'.  horny  laminae  of  "bar  ";  /".  ditto  of  wall; 
f",  lateral  aspect  of  a  lamina;  g,  tipper  or  inner  surface  of  horny  sole;  It.  junc- 
tion of  horny  lamina1  with  the  sole  (the  "white  line"):  /'.  toe-stay  at  middle  of 
toe;  k,  upper  or  inner  surface  of  horny  frog:  /.  frog-stay:  id.  cavity  correspond- 
ing to  a  branch  of  the  frog;  n,  ditto,  corresponding  to  body  of  frog. 


618 


COMPARATIVE  PHYSIOLOGY. 


terminations  of  the  common  extensor  and  the  perforans  ten- 
dons ;  the  lateral  cartilages  ;  a  certain  amount  of  connective 
and  fatty  tissue  ;  the  hoof-secreting  mechanism,  together  with 
the  hlood- vessels,  nerves,  lymphatics,  etc.,  essential  for  all  parts. 

The  relative  size  and  position  of  parts  may  be  gathered  from 
the  accompanying  cuts.  The  lateral  cartilages  belong  to  the 
class  known  as  fibro-cartilage,  acting,  no  doubt,  as  perfect 
buffers  ;  and  as  springs  must  be  of  no  small  assistance  in  loco- 
motion. 

The  horny  matter  of  the  foot  (hoof)  owes  its  formation  to 
the  cells  of  a  tissue  bearing  various  names  in  different  regions, 


h  7    k 


Fig.  466. 

Fig.  465.— Plantar  or  ground  surface  of  a  hoof;  right  foot.  The  interval  from  a  to  a 
represents  the  toe;  from  a  to  6,6,  outside  and  inside  quarters;  c,  o,  commence- 
ment of  bars;  d,  d,  inflexions  of  wall  at  heels  or  "  buttresses  ";  «,  lateral  lacuna; 
f,ff,  sole;  g,  white  line;  g",  ditto,  between  sole  and  bar;  h,  body  of  frog;  i, 
branch  of  frog;  k,  k,  glomes,  or  heels  of  frog;  I,  median  lacuna. 

Fig.  466. — Horn  cells  from  sole  of  hoof,  a,  young- cells  from  upper  surface  of  sole; 
b,  cells  from  lower  surface,  or  dead  horn  of  sole. 

but  consisting  of  a  basis  of  fibrous  tissue  abounding  in  blood- 
vessels and  nerves.  The  vessels  from  their  arrangement  have 
determined  the  names  given  to  the  formative  tissue,  such  as 
villosities,  villi,  velvety  tissue,  vascular  laminae,  etc.  It  can  not, 
however,  be  too  well  borne  in  mind  that  these  structures  are 
after  all,  only  modified  corium  (Fig.  371). 

Just  as  the  epidermis,  with  its  numerous  layers,  arises  from 
a  modification  of  cells  in  the  lower  layers,  resting  on  the  vascu- 
lar villi  of  the  corium,  so  the  hoof  owes  its  origin  to  a  similar 
source.  Thus  from  the  velvety  tissue  is  formed  the  sole  and 
frog  ;  from  the  perioplic  ring,  the  periople ;  and  from  the  coro- 
nary cushion,  the  wall  (see  figures). 


LOCOMOTION. 


619 


The  arrangement  of  the  horn-tubes,  the  horny  laminae  (Figs. 
467,  468),  and  the  horn-cells  is  admirably  adapted  to  form  a 
somewhat  yielding  yet  very  resisting  structure. 


Fig.  40r.— Horizontal  section  of  junction  of  wall  with  sole  of  hoof.  a.  wall  with  its 
horn-tubes:  b.  b,  horny  laminae  projecting  from  wall:  c.c.  horn-tubes  formed  by 
terminal  villi  of  vascular  laminae,  the  horn  surrounding  them  and  occupying  the 
spaces  between  the  horny  lamina  constituting  the  "white  line";  d,  horny  sole 
with  its  tnbes. 

Regarded  from  a  mechanical  point  of  view,  for  speed  a 
quadruped  requires  rather  long  limbs,  so  set  on  a  somewhat 
rigid  trunk  as  to  allow  of  a  long  as  well  as  a  rapidly  repeated 
stride,  without  undue  concussion  to  either  of  the  more  rigid 


Fig.  488. — Horizontal  section  of  wall  and  horny  and  vascular  laminre  to  show  junction 
of  latter  and  laminellse.  a,  inner  portion  of  wall  with  laminae  arising  from  it:  b. 
vascular  laminre;  c,  horny  lamina  of  average  length;  c'.  <•'.  unusually  short  lami- 
nae;  c",c",  laminellaa  on  the  sides  of  the  horny  laminae;  d.  vascula  lamina  passing 
between  two  horny  ditto;  d',  vascular  lamina  passing  between  three  horny  lami- 
nae; >/".  lateral  laminellaa;  e,e,  arteries  of  vascular  lamina  which  have  been  in- 
jected. 

cortical  parts.  In  the  horse  the  fore-limbs  are  not  attached  to 
the  trunk  by  osseous  connections,  but  the  animal  may  be  said 
to  be  slung  between  its  fore-limbs,  all  connections  with  the 
trunk  being  by  soft  parts,  as  muscles,  tendons,  and  ligaments. 


620 


COMPARATIVE  PHYSIOLOGY. 


The  advantages  of  such  an  ar- 
l'angenient,  to  an  animal  in  which 
a  great  deal  of  forward-pitching 
movement  occurs,  in  breaking 
shocks  are  evident.  The  length- 
ened metatarsals  and  phalanges 
are  accompanied  by  a  very  per- 
fect bracing  of  joints  by  liga- 
ments and  tendons  below,  while 
the  shoulder  is  strengthened  and 
bound  to  the  trunk  by  numerous 
muscles,  so  that  the  whole,  in 
neatness,  strength,  and  other 
qualities  required  in  a  fleet  ani- 
mal, is,  especially  when  taken  in 
connection  with  the  feet,  an  ex- 
ample of  marvelous  adaptation 
to  conditions  to  be  constantly 
met,  aided  in  the  wild  species  by 
natural  selection,  and  in  our  do- 
mestic varieties  by  artificial  se- 
lection. 

An  examination  of  Fig.  470 
will  show  the  several  levers 
(bones)  and  the  muscles  acting  on 
them  in  one  main  movement  of 
the  fore-limb. 

The  hind-limbs  are  in  all  gaits 
of  the  animal  its  main  propellers, 
and  these  are  in  bony  connection 
with  the  pelvis. 

Fig.  400. — Extern.il  muscles  of  right  anterior 
limb  (Chauveau),  1, 1,  long  abductor  of 
arm;  V.  its  humeral  insertion:  2,  super- 
spinatus  ;  3,  subspinatus;  3',  its  tendon 
of  insertion  ;  4,  short  abductor  of  arm; 
5,  biceps;  6,  anterior  brachialis;  7,  large 
extensor  of  forearm;  8,  short  extensor 
of  forearm;  0,  anconeus;  11,  anterior  ex- 
tensor of  metacarpus;  W,  its  tendon;  12, 
aponeurosis,  separating  that  muscle  from 
anterior  brachialis;  13,  oblique  extensor 
of  metacarpus;  14,  anterior  extensor  of 
phalanges;  II',  its  principal  tendon;  15,  small  tendinous  branch  it  furnishes  to 
lateral  extensor;  16,  lateral  extensor  of  phalanges;  16',  its  tendon;  17,  fibrous 
band  it  receives  from  carpus;  in,  external  flexor  of  metacarpus;  19,  its  metacar- 
pal tendon;  2D.  its  supracarpal  tendon;  21.  ulnar  portion  of  perforans;  22,  tendon 
of  perforans;  23,  its  carpal  ligament;  21,  its  re-enforcing  phalangeal  sheath;  25, 
tendon  of  the  perforans, 


LOCOMOTION. 


621 


It  will  not  be  forgotten  that  in  joints  the  insheaihing  carti- 
lages (sometimes  others  more  or  less  free),  the  synovial  fluid, 
etc.,  all  tend  to  diminish  friction  and  lessen 
concussion. 

We  shall  now  describe  the  principal  gaits 
of  the  horse  in  a  somewhat  synoptical  way. 
In   each  gait  we  have  to   consider  the 
relative  position  of  the  four  limbs,  the 
duration  of  each  phase  in  the  move- 
ment, the  length  of  the  stride,  its 
rate,  etc.     Much  that  applies  to 
the  horse  holds  good,  of  course, 
of  other  quadrupeds. 

In  every  gait  each  leg  passes 
from  a  condition  of  flexion  to 
one  of  extension,  the  degree  be- 
ing dependent  on  the  speed  or, 
more  correctly,  the  effort  of  the 
animal  to  attain  high  speed  or 


reverse. 


Pig.  470. — Internal  aspect  of  left  an- 
terior limb  (Chauveau).  1,  pro- 
longing cartilage  of  scapula  ;  2, 
inner  surface  of  scapula;  3,  sub- 
scapulars; 4.  adductor  of  fore- 
arm, or  portion  of  caput  mag- 
num; 7.  large  extensor  of  fore- 
arm, other  portion  of  caput  mag- 
num; 8,  middle  extensor,  or  ca- 
put medium:  11.  humeralis  exter- 
nns,  or  short  flexor  of  forearm; 
10,  coraco-humeralis;  11,  upper 
extremity  of  humerus ;  12,  co- 
raco-radialis.    or   flexor  brachii  ; 

13,  lower  extremity  of  humerus; 

14.  brachial  fascia  ;  15.  anterior 
extensor  of  metacarpus,  or  ex- 
tensor metacarpi  magnus  :  16, 
belly  and  aponeurotic  termina- 
tion of  flexor  brachii;  17.  ulna: 
18,  ulnaris  accessorius.  or  oblique 
flexor  of  metacarpus:  11),  inter- 
na! flexor  of  metacarpus,  or  epicondylo-metacarpus;  20.  ther  set  is  wholly 
radius:  21,  tendon  of  oblique  extensor:  22.  large  meta- 

carpal-bone  :  33,  flexor  tendons  of  foot :  24,  suspensory    relaxed.  xhe 

ligament;  25,  internal  rudimentary  metacarpal  bone;  26.  ,-■  -i  i 

extensor   tendon    of    foot;    27,   metucarpo  -  phalangeal     more  UlOl'OUgim 
sheath;  28,  lateral  cartilages  of  foot;  29,  podophylhe.  muscular     move- 

ments are  studied  the  more  complex,  so  far  as  the  use  of  muscles 
is  concerned,  are  they  found  to  be,  a  fact  which  is  illustrated 
when  even  a  single  muscle  is  weakened  or  paralyzed. 


When  the  foot 
rests  upon  the 
ground  before 
the  limb  is  re- 
moved, it  de- 
scribes the  arc 
of  a  circle,  or  os- 
cillates like  a 
pendulum  so  that 
the  flexors  and 
extensors  are 
used  alternately 
more  and  less ; 
==^=-*  though  in  all 
movements  it  is 
likely    that   nei- 


622 


COMPARATIVE   PHYSIOLOGY. 


Walking. — In  tliis  gait  the  body  rests  on  diagonal  feet  alter- 
nately with  the  two  of  the  same  side  ;  the  center  of  gravity 
being  shifted  to  one  side,  then  returned  to  its  original  position, 
to  be  moved  next  to  the  opposite  side.  In  drawing  heavy  loads 
the  body  is  supported  on  three  limbs.  The  rate  of  movement  is 
one  to  two  metres  per  second. 

Amble. — In  this  mode  of  progression,  most  common  in  the 


Fig.  471— Movements  (oscillation)  of  an  extended  hind-leg  (Colin).  The  hip-jomt 
describes  the  arc  of  a  circle,  ABC,  while  the  foot  is  on  the  ground,  the  lines  A D, 
B  D,  and  C  D  representing  the  changing  axis  of  the  hind-leg. 

giraffe  and  camel  tribe,  occasional  in  ruminants  and  solipeds, 
the  body  is  supported  by  the  two  legs  on  the  same  side,  as  in  the 
walk,  but  the  two  opposite  legs  are  elevated  simultaneously  and 
not  separately.  In  horses  this  gait  is  often  termed  pacing,  and 
is  frequently  very  fast.  Only  two  strokes  of  the  feet  are  heard 
in  this  gait. 

In  racking  the  hind-leg  leaves  the  ground  sooner  than  the 
corresponding  fore-leg,  hence  four  strokes  of  the  feet  are  heard. 

The  Trot.— The  diagonal  feet  act  together,  two  strokes  of  the 
feet  being  heard  at  each  complete  step.  In  the  fast  trot  there 
is  an  interval  in  which  all  four  feet  are  in  the  air.  The  hind- 
feet  strike  the  ground  in  front  of  the  fore-feet.     The  speed  in 


LOCOMOTION. 


623 


the  fast  trot  may  reach  from  eight  to  twelve  metres  per  sec- 
ond. 

The  Gallop. — The  gallop  may  be  regarded  as  a  series  of 


Fig.  472. — Movements  of  fore-limbs  of  horse  (Colin).  While  one  fore-leg  is  describine 
the  movements  figured  above  the  other  acts  as  a  support.  While  the  right  fore- 
foot describes  the  arc  gh,  the  left  shoulder  describes  the  arc  a'  b'  c',  owing  to  the 
impulse  from  extension  of  the  hind  legs.  The  center  of  gravity  is  advanced  from 
m  to  n,  the  left  leg  in  one  complete  step  occupying  the  six  positions  indicated  at 
abed  ef. 

jumps  in  which  the  hind-legs  take  the  greater  part,  though  as 
in  all  gaits  the  fore-legs  are  not  only  supporters  but  propellers. 


Fig.  473.— Various  positions  of  the  limbs  in  the  trot  (Colin). 

In  the  perfect  gallop  only  two  strokes  of  the  feet  are  heard;  in 
the  canter  or  slow  gallop  four,  in  the  ordinary  gallop  three. 
According  as  the  one  or  other  hind-leg  is  extended  farthest 
behind  the  body  the  gallop  is  termed  right-handed  or  left- 
handed. 


624 


COMPARATIVE   PHYSIOLOGY. 


In  the  fastest  gallop  the  length  of  stride  may  amount  to  six 
to  seven  metres,  and  the  speed  to  twelve  to  fifteen  metres  per 
second.  In  such  a  rapid  gait  the  contact  of  the  one  hind  foot 
produces  a  sound  lengthened  by  the  rapid  impact  of  the  fellow- 
foot.  The  same  applies  to  the  fore-feet,  hence  only  two  sounds, 
while  in  the  other  varieties  of  this  gait  the  interval  between  the 
impacts  is  sufficient  to  allow  of  three,  or  it  may  be  four  sounds. 

The  accompanying  plate,  constructed  by  the  help  of  instan- 
taneous photography,  illustrates  the  different  positions  of  a 
horse  in  the  gallop. 

Sloping  shoulder-blades  and  well-bent  stifle-joints  are  gener- 
ally recognized  as  of  great  importance  to  an  animal  intended 
for  high  speed,  and  these  are  commonly  to  be  met  with  in  the 


Fig.  474.— Various  positions  in  the  trot  (Colin) 


fleetest  of  horses,  dogs,  and  other  quadrupeds  (Fig.  452).  It 
may  be  seen  that  such  an  arrangement  permits  of  a  length- 
ened stride  being  taken  with  ease,  tends  to  reduce  concussion, 
and  adds  to  beauty  of  form.  To  this  must,  in  part  at  all  events, 
be  attributed  the  grace  of  form  and  fleetness  of  the  race-horse 
and  the  greyhound,  not  to  mention  wild  animals. 

A  horse  for  heavy-draught  purposes  requires  great  muscular 
power,  which  in  turn  implies  a  strongly  developed  osseous  sys- 
tem; and  in  order  that  this  may  be  attained  some  of  those 
principles  on  which  speed  depends  must  be  subordinated  to 
those  involved  in  strength.  As  is  well  known,  the  cart-horse 
and  race-horse,  the  mastiff  and  the  greyhound,  are  opposites  in 
build  and  capacity  for  speed.  However,  between  these  extreme 
forms  there  are  many  others  of  an  intermediate  character,  as 
the  hunter,  roadster,  etc.    When  famous  race-horses  are  studied, 


626  COMPARATIVE  PHYSIOLOGY. 

while  the  form  of  the  animal  generally  agrees  with  what  would 
have  been  expected  on  mechanical  principles  it  is  a  fact  that 
some  of  the  fleetest  horses  that  have  ever  run  on  the  course  have 
not  in  all  respects  been  built  in  conformity  with  them.  But 
it  is  to  be  remembered  that  a  vital  mechanism  differs  from  all 
others  in  that  the  whole  consists  of  parts  dependent  not  only  as 
one  portion  of  any  machine  is  on  the  other,  but  that  every  part 
is  energized  and  directed  by  a  governing  nervous  system ;  that 
every  cell  is  being  in  a  sense  constantly  renewed,  so  that  the 
comparison  between  any  ordinary  mechanism  and  the  body  of  a 
living  animal  holds  only  to  a  limited  extent.  Moreover,  apart 
from  peculiarities  in  the  muscles  of  animals,  to  which  atten- 
tion has  been  drawn  (page  205),  it  is  well  to  bear  in  mind  that  not 
only  every  animal,  but  every  tissue  has  its  own  functional  indi- 
viduality ;  and  to  this  especially  (as  exemplified  in  the  most  im- 
portant of  all  the  tissues,  the  nervous)  must  we  attribute  the 
undoubted  fact  that  the  speed,  endurance,  etc.,  of  animals  can 
not  be  explained  on  mechanical  principles  alone — a  truth  to 
which  too  little  attention  has  hitherto  been  drawn.  These 
principles  have,  however,  been  unconsciously  recognized  prac- 
tically, hence  the  great  attention  paid  by  breeders  to  using  ani- 
mals for  stock  purposes  that  have  actually  shown  merit  by  their 
performances. 

Evolution. — It  is  noteworthy  that  with  almost  all  quadru- 
peds the  gallop  is  the  natural  method  for  rapid  propulsion.  In 
all  animals,  either  bred  by  man  to  attain  great  speed,  as  the 
race-horse  and  greyhound,  or  those  that  have  become  so  by  the 
process  of  natural  selection,  the  entire  conformation  of  the 
body  has  been  modified  in  harmony  with  the  changes  that  have 
taken  place  in  the  legs  and  feet.  This  is  seen  in  the  greyhound 
among  domestic  animals,  and  in  the  wild  deer  of  the  plain  and 
forest.  Such  instances  illustrate  not  only  the  principle  of 
natural  selection  as  a  whole,  but  the  subordinate  one  of  corre- 
lated growth. 

Any  one  observing  the  modes  of  locomotion  of  quadrupeds, 
especially  horses  and  dogs,  will  perceive  the  advantages  of  the 
four-legged  arrangement.  Not  only  is  there  a  variety  of  modes 
of  progression,  as  walking,  trotting,  galloping,  cantering,  the 
alternations  of  which  permit  of  rest  to  certain  groups  of  mus- 
cles, with  their  corresponding  nervous  connections,  etc.,  but  on 
occasion  some  of  these  animals  can  progress  fairly  well  with 
three  legs.     Sometimes  it  may  also  be  noticed  that  a  horse  that 


LOCOMOTION.  627 

prefers  one  gait,  as  pacing,  for  his  easy,  slow  movements,  will 
break  into  a  trot  when  pushed  to  a  higher  rate  of  speed. 

Trotting  can  not  be  considered  the  natural  gait  for  high 
speed  in  the  horse,  yet,  by  a  process  of  "artificial  selection" 
(by  man)  from  horses  that  have  shown  capacity  for  great  speed 
by  this  mode  of  progression,  strains  of  racers  have  been  bred, 
showing  that  even  an  acquired  mode  of  locomotion  may  be 
hereditary ;  while  that  galloping  is  the  more  natural  mode  of 
locomotion  of  the  horse  is  evident,  among  other  things,  by  the 
tendency  of  even  the  best  trotting  racers  to  break  into  a  gallop 
when  unduly  pushed — an  instance  also  of  an  hereditary  tend- 
ency of  more  ancient  origin  prevailing  over  one  more  recent. 

The  bipedal  modes  of  progression  of  birds  are  naturally  very 
like  those  of  man. 


INDEX 


Abductor  or  sixth  nerve,  582. 

Abnormal  urine,  421. 

Accelerator  nerves  of  heart,  258. 

Accommodation  of  eye,  532. 

Action  of  mammalian  heart,  222. 

Affections  of  retina,  540. 

Afferent  fibers,  583. 

After-images,  etc.,  543. 

Alimentary     canal     of     vertebrate, 
331. 

Allantoic,  77. 

Allantoic  cavity,  80. 

Albumins,  145. 
derived,  145. 

Alterations  in  size  of  pupil,  533. 

Amble,  624. 

Amnion,  76. 

Amoeba,  13. 

Amylolytic  action  of  saliva,  297. 

Animal  body,  28. 

Animal  foods  (table),  277. 

Animal  heat,  445. 

Animals  deprived  of  cerebrum,  482. 

Anaemia,  117. 

Anomalies  of  refraction,  536. 

Apncca,  395. 

Apparatus   used    for   stimulation   of 
muscle,  179. 
for  transmission  of  muscular  move- 
ments by  tambours,  182. 

Asphyxia,  respiration  and  circulation 
in,  399. 

Auditory  ossicles,  559. 


Auditory  impulses,  565. 
sensations,  etc.,  567. 
Automatism,  nervous  system,  211. 
Automatic  functions  of  spinal  cord, 
475. 

Bacteria,  18. 
Barking,  402. 

Bawling,  neighing,  braying.  403. 
Beat  of  the  heart  and  its  modifica- 
tions, 248. 
Bell-animalcule,  21. 
Bile,  digestive  action  of,  303  . 

salts,  302. 

pigments,  302. 
Biology,  general,  1. 

table,  4. 
Blastodermic  vesicle,  78. 
Blood,  154. 

cells,  158. 

cells,  decline,  and  death,  1 60. 

chemical  composition  of,  160. 

pressure,  223-227. 

flow,  227. 
Bone,  604. 
Botany,  4. 
Brain.  481. 

Capillaries.  264. 
Carbon-dioxide  of  blood,  389. 
Carbohydrates,  146. 
Cardiac  movements,  231. 
sounds,  234. 


630 


COMPARATIVE   PHYSIOLOGY. 


Cartilage,  609. 

Causes  of  the  sounds  of  the  heart,  235. 

Cell,  5. 

the  male,  61. 
Cellulose,  9. 
Cerebellum,  508. 
Cerebral  cortex,  497. 
Cerebro-spinal  system  of  nerves,  580. 
and  sympathetic  systems,  relations 
of,  588. 
Certain  tissues,  603. 
Characteristics  of   proteids,  general, 
144. 
of  blood-flow,  226. 
of   secretion   of    different  glands, 
298. 
Changes  in   muscle   during    contrac- 
tion, 189. 
produced    in  food   in    alimentary 

canal,  354. 
in  circulation  after  birth,  129. 
Chemical  constitution  of  animal  body, 
142. 
changes  in  muscle,  189. 
composition  of  blood,  160. 
Chemistry  of  unicellular  plants,  9. 
Chondrin,  145. 
Chorion,  79. 
Chronographs,  176. 
Ciliary  movements,  179. 

(ophthalmic    lenticular),    ganglion, 
584. 
Circulation  of  blood,  214. 
in  mammal,  219, 
under  microscope,  224. 
in  brain,  500. 
Circumstances   influencing   character 
of  muscular  and  nervous  activ- 
ity, 199. 
Classification  of  animal  kingdom,  34. 

of  proteids,  145. 
Clinical   and    pathological    re   blood, 

167. 
Coagulation  of  blood,  163. 


Coitus,  129. 

Color-vision,  543. 

Comparative  re  blood,  154,  172. 

unstriped  muscle,  202. 

blood-pr,essure,  224. 

cardiac  pulsation,  240. 

circulation,  244,  267. 

digestion,  280,  310,  357. 

metabolism,  436. 

diet,  438. 

digestive  juices,  298. 

digestive  organs,  324,  337. 

bile,  303. 

feeding  experiments,  441. 

fats  and  carbohydrates,  442. 

animal  heat,  445. 

spinal  cord,  477. 

cerebral  convolutions,  485. 

muscular  sense,  524. 

vision,  536-551. 

hearing,  568. 

senses  of  smell  and  taste,  674,  578. 

voice,  598. 

locomotion,  614. 

swallowing,  336. 

vomiting,  340. 

movements  of  lymph,  344. 

respiration,  376,  398. 

haemoglobin,  389. 

respiratory  movements,  402. 

respiration  by  skin,  412. 

perspiration,  413. 

expulsion  of  urine,  426. 
Comparison  of  inspired  and  expired 

air,  382. 
Composition  of  serum,  131. 

of  corpuscles,  162. 

of  milk,  275. 
Conclusions  re  unicellular  plants,  10, 

protococcus,  12. 

unicellular  animals,  15. 

nervous  system,  212. 

heart,  257. 

salivary  secretion,  314. 


INDEX, 


631 


Condiments,  278. 
Connective  tissue,  603. 
Contractile  tissues,  1*71. 
Connection  of  one  part  of  brain  with 

another,  491. 
Construction  of  fat,  432. 
Conditions  under  which  gases  exist 

in  blood,  384. 
Co-ordination  of  two  eyes  in  vision, 

544. 
Coughing,  401. 
Corpuscles,  156. 

action  of  the,  22Y. 
Corpora  quadrigemina,  506. 
Corpus  striatum  and  optic  thalamus, 

504. 
Cranial  nerves,  580. 
Crying,  402. 

Decussation,  4*71. 

Defecation,  338. 

Deglutition,  333. 

Dentition  of  domestic  animals  (table), 

290-296. 
Development  of  embryo,  95. 

of  vascular  system  in  vertebrates, 
108. 

of  urogenital  system,  112. 
Dextrin,  146. 
Diet,  437-439. 

effects  of  gelatin  in,  441. 

effects  of  salts,  water,  etc.,  443. 
Digestion  of  food,  274. 
Digestive  juices,  297. 

action  of  bile,  303. 

organs,  movements  of,  332. 
Dioptrics  of  vision,  531. 
Discoidal  placenta,  83. 
Domesticated  animals,  47. 
Dyspnoea,  396. 

Effects  of  gelatin  in  diet,  411. 
Efferent  nerve-fibers,  583. 
Entrance  and  exit  of  air,  370. 


Elasticity  of  muscle,  189. 

Elastin,  145. 

Elastic  tissues,  604. 

Electrical  phenomena  of  muscle,  191. 

organs,  197. 
Embryological  re  digestion,  279. 

brain,  510. 

vision,  528. 
Embryo,  development  of,  95. 
Embryology,  applied  to  evolution,  45. 
Embryonic  membrane  of  birds,  74. 
Endocardiac  pressure,  236. 
Energy  of  animal  body,  443. 
Epiblast,  98. 
Epithelium,  7. 
Evolution,  42. 

re  reproduction,  93. 

circulation,  268. 

digestion,  363. 

respiration,  404. 

metabolism,  450. 

spinal  cord,  478. 

brain,  511. 

vision,  552. 

hearing,  571. 

voice,  600. 

locomotion,  627. 
Estimation  of  size,   etc.,  of  objects, 

548. 
Excretory  function  of  skin,  411. 
Excretion  of  perspiration,  412. 

by  the  kidney,  415. 
Experimental  facts,  185. 

re  nervous  system,  210. 

digestion,  305. 

spinal  nerves,  580. 
Eustachian  tube,  562. 
Eye,  accommodation  of,  532. 

optical  imperfections  of,  536. 

protective  mechanisms  of,  549. 

Facial  nerve,  581. 

and  laryngeal  respiration,  374. 
Fat,  construction  of,  432. 


632 


COMPARATIVE  PHYSIOLOGY. 


Fats,  145.  . 

and  carbohydrates,  442. 
Fatigue,  199. 

Feeding  experiments,  439. 
Features  of  an  arterial  pulse  tracing, 

242. 
Fertilization  of  ovum,  63. 
Fibrin,  145,  164. 
Faeces,  352. 
Foetal  circulation,  125. 

later  stages  of,  109. 

membranes  of  mammals,  78. 
Food,  digestion  of,  274. 

stuffs,  274. 
Foods,  animal,  table  of,  277. 

vegetable,  table  of,  277. 
Foreign  gases  in  respiration,  391. 
Forced  movements,  484. 
Fossil  and  existing  species,  46. 
Fresh-water  polyps,  23. 
Fungi,  15. 

Functional  variations,  204. 
Functions   of   cerebral  convolutions, 
485. 

of  other  portions  of  brain,  504. 

Gastrula,  68. 

Gastric  juice,  299. 

Gelatin,  145. 

Geographical  distribution,  46. 

Globulins,  145. 

Glosso-pharyngeal    or    ninth    nerve, 

585. 
Glycogen,  428. 
uses  of,  429. 
Glycocholic  acid,  302. 
Gout,  139. 
Graafian  follicle,  58. 
Graphic  method  and  study  of  muscle 

physiology,  176. 
Gustatory  fibers,  582. 

Haemoglobin  and  its  derivatives,  385. 
Hearing,  557. 


Heart,  231. 

of  various  animals,  245,  246. 

beat  in  cold-blooded  animals,  250. 

causation  of  beat  of,  253. 

influence  of  vagus  nerve  on,  253. 

accelerator  nerves  of,  258. 

in  relation  to  blood-pressure,  260. 
Hen's  egg,  69. 
Hiccough,  402. 
History  of  blood-cells,  158. 
Hydra,  26. 
Hypoblast,  98. 
Hypertrophy,  265. 
Hypnotism,  502. 
Hypoglossal  or  twelfth  nerve,  588. 

Impulse  of  heart,  232. 
Influence  of  blood-supply,  199. 

of  temperature,  201. 

of  vagus  nerve  upon  heart,  253. 

of  condition  of  blood  in  respira- 
tion, 393. 

of  respiration  on  circulation,  396. 

of  nervous  system  on  metabolism, 
452. 
Inhibition  of  reflexes,  469. 
Inorganic  food-stuffs,  144,  274. 
Inosit,  145. 
Inorganic  salts,  420. 
Instincts,  42. 
Investigation    of    heart  -  beat    from 

within,  233. 
Intestinal  movements,  337. 
Irritability  of  muscle  and  nerve,  175. 

Juices,  digestive,  297. 

Keratin,  145. 

Lactose,  146. 

Law  of  periodicity  or  rhythm  in  na- 
ture, 37. 

of  habit,  41,405. 
of  rhythm,  269. 


INDEX. 


633 


Laws  of  retinal  stimulation,  541. 

Laughing,  401. 

Living  things,  2. 

Living  and  lifeless  matter,  32. 

Lymphatic  system,  342. 

Lymph  and  chyle,  343. 

Locomotion,  610. 

Maltose,  146. 

Mammalian  heart,  215,  222. 
Man's  place  in  animal  kingdom,  36. 
Medulla  oblongata,  509. 
Membrana  tympani,  558. 
Mesoblast,  98. 
Metadiseoidal  placenta,  84. 
Metabolism,  27,  428. 

of  liver,  428. 

of  spleen,  429. 

influence   of   nervous    system    on, 
452. 

summary  of,  45S. 
Metazoa,  5,  53. 
Milk,  composition  of,  275. 

sugar,  276. 
Mimicry,  45. 
Molds,  15. 
Morphology  of  unicellular  plants,  9. 

of  protococcus,  12. 

of  unicellular  animals,  13. 

applied  to  evolution,  45. 
Motor  oculi  nerve,  581. 
Movements  of  digestive  organs,  332. 

stomach,  336. 

lymph,  344. 
Mucin,  145. 
Mucor  mucedo,  17. 
Mullerian  duct,  115. 
Multicellular  organisms,  23. 
Muscular  contraction,  185. 
Muscle  tone,  189. 
Muscular  work,  198. 
Muscles  of  respiration,  373. 

of  middle  ear,  561. 
Muscular  sense,  524. 


Nasal   or   spheno-palatine    ganglion, 

584. 
Nature  of  act  of  secretion,  318. 
Nerve-cells,  209. 

supply  (voice),  596. 
Nervous  mechanism,  134. 

system,  208. 

inhibition,  212. 

system  in  relation  to  heart,  249. 

supply — digestion,  333. 

system  in  relation   to  respiration, 
393. 
Nitrogen  of  blood,  389. 
Nitrogenous  metabolites,  147. 

crystalline  bodies,  420. 

equilibrium,  440. 
Non-nitrogenous  metabolites,  147. 

organic  bodies,  420. 
Non-crystalline  bodies,  145. 
Notochord,  99. 
Nucleus,  5.  i ' 

Nucleolus,  5. 
Nuclear  division,  6. 
Nuclein,  145. 
Nutrition  of  ovum,  123. 

Ocular  movements,  544. 

(Estrum,  121. 

Optical  imperfections  of  the  eye,  536. 

Origin  of  forms  of  life,  42. 

of  spermatozoon,  61. 

of  fowl's  egg,  70. 
Organic  evolution  reconsidered,  137. 

food-stuffs,  144-274. 
Otic  ganglion,  585. 
Ovulation,  120. 
Ovum,  55. 

origin  and  development  of,  57. 

changes  in,  58. 
Oxy-ha3moglobin,  386. 

Paleontology,  46. 
Pancreatic  juice,  305. 
Parasitic  organisms,  15. 


634 


COMPARATIVE  PHYSIOLOGY. 


Parturition,  128. 
Pathological  re  food-stuffs,  147. 
muscle,  197. 
circulation,  244,  265. 
bile,  318. 

stomach,  330,  337. 
lymphatics,  352. 
faeces,  354. 
digestion,  359. 
respiration,  379,  401,  403. 
skin,  411. 
urine,  423. 

expulsion  of  urine,  426. 
metabolism,  435,  443. 
temperature,  449. 
spinal  cord,  471. 
brain,  508,  509. 
muscular  sense,  524. 
vision,  536,  554. 
hearing,  563. 
spinal  nerves,  580. 
third  and  other  nerves,  582,  585, 

587,  588. 
voice,  598. 
Peculiar  respiratory  movements,  401. 
Peptones,  145. 
Pendulum  myograph,  184. 
Perspiration,  excretion  of,  412. 
Periods  of  gestation,  127. 
Pfliigcr's  monograph,  181. 
Physiology  of  unicellular  plants,  10. 
of  protococcus,  12. 
unicellular  auimals,  13. 
nerves,  195. 
Physiological  aspects  of  development, 
118. 
research  and  reasoning,  148. 
Placenta,  82. 
discoidal,  83. 
metadiscoidal,  84. 
zonary,  89. 
diffuse,  89. 
polycotyledonary,  89. 
Simple,  90. 


Placenta  multiple,  90. 

microscopic  structure  of,  90. 

Pneumogastric  nerve,  586. 
Polyps,  23. 

Pressure,  endocardiac,  236. 
Pressure  sensations,  522. 
Proteids,  general   characteristics  of, 
144. 

of  milk,  276. 
Proteus  animalcule,  13. 
Protococcus,  11. 
Protozoa,  5,  53. 
Protective  and  excretory  function  of 

skin,  40S. 
Protective  mechanism  of  the  eye,  549. 
Protoplasm,  3. 
Proximate  principles,  144. 
Pulse,  241. 

the  venous,  244. 
Psychological  aspects  of  vision,  543. 

Quantity  and  distribution  of  blood, 
163. 

of  blood,  influence  on  blood-press- 
ure, 261. 

of  air  respired,  378. 

Reflex  functions  of  the  spinal  cord, 

466. 
Regulation  of  temperature,  447. 
Relations  of  cerebro-spinal  and  sym- 
pathetic symptoms,  588. 
Relative  value  of  food-stuffs,  444. 

time  occupied  by  cardiac  cycle,  237. 
Removal  of  digested  products  from 

the  alimentary  canal,  341. 
Reproduction,  51. 
Retinal  stimulation,  laws  of,  541. 
Respiratory  system,  366. 

rhythm,  379. 

sounds,  81. 
Respiration,  muscles  of,  373. 

facial  and  laryngeal,  374. 

types  of,  375. 

in  blood,  383. 


INDEX. 


635 


Respiration  in  tissues,  391. 

Respiration   and    circulation    in    as- 
phyxia, 399. 
in  mammal,  405. 
by  skin,  412. 

Rigor  mortis,  206. 

Rhythm,  37. 
law  of,  269. 

Rudimentary  organs,  46. 

Salts,  144,  276. 

inorganic,  420. 
Saliva,  297. 

amylolytic  action  of,  297. 
Scar-tissue,  139. 
Serum,  composition  of,  161. 
Secretion  as  a  physiological  process, 

of  salivary  glands,  311. 

by  stomach,  315,  321. 

of  bile  and  pancreatic  juice,  316. 

nature  of  the  act,  318. 

of  mine,  421. 
Secretory  fibers,  582. 
Segmentationand  subsequent  changes, 

64. 
Self-digestion    of    digestive    organs, 

323. 
Sense  organs,  31. 
Sexual  selection,  43. 
Separation   of   muscle   from    central 

nervous  system,  201. 
Semicircular  canals,  function  of,  484. 
Sighing,  402. 
Senses,  general  remarks,  516. 

of  smell  and  taste,  573. 
Skin  as  an  organ  of  sense,  520. 
Smell,  573. 
Sleep,  501. 
Sobbing,  402. 
Solidity,  548. 
Soap,  146. 
Sneezing,  402. 
Special  considerations  re  muscle,  208. 

circulation,  265. 


Special   considerations   re   digestion, 
359. 

metabolism,  450. 

spinal  cord,  477. 

brain,  510. 

vision,  551. 

hearing,  568. 

voice,  600. 
Specific  gravity  of  urine,  419. 
Spermatozoa,  61. 
Spbygmograph,  243. 
Spinal  cord,  general,  461. 

reflex  functions  of,  466. 

as  a  conductor  of  impulses,  469. 

automatic  functions  of,  475. 

nerves,  579. 

accessory,  587. 
Sporangia,  17. 
Starches,  147. 

Study  of  metabolic  processes,  436. 
Submaxillary  ganglion,  585. 
Succus  entericus,  307. 
Sugars,  146. 
Summary  of  biology,  9. 

of  evolution,  50. 

of  reproduction,  93. 

development  of  the  embryo,  135. 

physiological  research,  etc.,  152. 

blood,  169. 

muscle  and  nerve,  205. 

circulation,  269. 

blood-cells,  158. 

digestion,  364. 

respiration,  405. 

perspiration,  413. 

urine,  426. 

metabolism,  458. 

voice,  001. 
Synoptical  re  spinal  cord,  479. 

brain,  513. 

skin,  525. 

vision,  564. 

hearing,  572. 
Swallowing,  335. 


636 


COMPARATIVE   PHYSIOLOGY. 


Tactile  sensibility,  523. 
Tambour  of  Marey,  183. 
Taste,  575. 
Teeth,  2S4. 

structure  and  arrangement  of  the, 
286. 
Temperature,  regulation  of,  447. 
Tetanizing  key  of  Du  Bois-Reymond, 

181. 
Tetanic  contraction,  187. 
Thermal  changes  in  contracting  mus- 
cle, 195. 
sensations,  522. 
Tissues,  8. 
Trigeminus,  trifacial   or  fifth  nerve, 

583. 
Trochlear  or  fourth  nerve,  582. 
Trot,  624. 
Types  of  respiration,  375. 

Unicellular  plants,  9. 

animals,  13. 

with  differentiation  of  structure,  21. 
Unstriped  muscle,  202. 
Urea,  147. 

Urine  considered  physically  and  chem- 
ically, 419. 

abnormal,  421. 


Urine,  secretion  of,  421. 
expulsion  of,  424. 

Variations  in  cardiac  pulsation,  239. 

of  average  tempei'ature,  446. 
Vasomotor  nerves,  262. 
Vegetable  foods  (table),  277. 
Velocity  of  blood  and    blood-press- 
ure, 223. 
Venous  pulse,  244. 
Vision,  526. 

dioptrics  of,  531. 

psychological  aspects,  543. 
Visual  sensations,  538. 

angle,  542. 
Voice,  593. 
Vomiting,  339. 
Vorticella,  21. 

Walking,  623. 
Water,  443. 
Wolffian  duct,  LI 5. 
Work  of  the  heart,  238. 

Yeast,  9. 

cells,  10. 
Yawning,  402. 

Zoology,  4. 


THE   END. 


October,   IS93. 

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